Metallurgy in Space: Recent Results from ISS (The Minerals, Metals & Materials Series) 3030897834, 9783030897833

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The Minerals, Metals & Materials Series

Hans-Jörg Fecht Markus Mohr Editors

Metallurgy in Space Recent Results from ISS

The Minerals, Metals & Materials Series

The Minerals, Metals & Materials Series publications connect the global minerals, metals, and materials communities. They provide an opportunity to learn about the latest developments in the field and engage researchers, professionals, and students in discussions leading to further discovery. The series covers a full range of topics from metals to photonics and from material properties and structures to potential applications.

More information about this series at http://www.springer.com/series/15240

Hans-Jörg Fecht • Markus Mohr Editors

Metallurgy in Space Recent Results from ISS

Editors Hans-Jörg Fecht Institute of Functional Nanosystems Ulm University Ulm, Germany

Markus Mohr Institute of Functional Nanosystems Ulm University Ulm, Germany

Jointly published with The Minerals, Metals & Materials Society ISSN 2367-1181 ISSN 2367-1696 (electronic) The Minerals, Metals & Materials Series ISBN 978-3-030-89784-0 (eBook) ISBN 978-3-030-89783-3

https://doi.org/10.1007/978-3-030-89784-0 © The Minerals, Metals & Materials Society 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publishers, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publishers nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Foreword by Alexander Gerst

MacGyver Is Melting Metals in Space During my first mission to the International Space Station (ISS), called “Blue Dot,” it was one of my main tasks to install the electromagnetic levitator (EML) in ESA’s magnificent Columbus laboratory. This furnace represents one of the most impressive material science experiments in space. It can heat a metallic alloy to 2200  C and then cool and solidify it rapidly, while the molten droplet remains freely suspended and undisturbed in the experiment chamber. This is impossible to achieve on Earth. Material scientists study these free-floating droplets in order to obtain characteristic thermophysical properties of the molten alloy in its liquid state. These thermophysical property data are needed for the development of computer models that simulate industrial production processes on Earth. Consequently, this increases the production efficiency and the quality of end products and reduces the energy consumption in line with major global efforts of green energy. It is a good example for why we sometimes have to go to a strange and hostile place like space to improve things down here on Earth. As a result, the better material performance on Earth leads to reduced energy consumption and greenhouse gas emissions, for example, in advanced turbines for aircraft and land-based power generation. Other materials that were studied by scientists all around the world using the material science laboratory on board the ISS are, for example, high-strength steels, low-weight titanium-alloys for a wide range of applications from aerospace to biomedicine, new semiconductors for photovoltaics, and new metallic glass alloys designed for future rovers on the Moon and Mars. From this impressive list of potential benefits that can be obtained by operating a device like EML on ISS, you can probably estimate the “weight” on the shoulders of an astronaut who is given the complex task to install such a device. In contrary to the assumption of most humans on Earth, an astronaut’s biggest fear is not to end up in a v

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fireball during launch (even though admittedly that is also not a nice thought, it is relatively unlikely) but to make a mistake that endangers the success of the many invaluable experiments, designed and built by hundreds of engineers and scientists over the course of many years. To disappoint them would be a true failure for us. You can now probably imagine the feeling of terror that I had when, during the installation of EML, I realized that one of the launch fasteners that secured the hardware from the massive vibrations during its launch was stuck and couldn’t be removed. What was even worse, it prevented the final assembly of EML, and therefore it threatened the entire project – just a single tiny bolt, in a very difficult place to access. But what followed was one of the finest examples of what an international team of experts can achieve when they all work together. A crisis team was formed, consisting of our mission ground team, engineers, and me, the astronaut in orbit. In the following weeks, we went through various options of removing the stuck bolt and meticulously analyzed the risks of each approach. In the end, ground teams allowed me to go ahead with the option we called “MacGyver,” which, needless to say, was my favorite: I got the GO to cut off the bolt with a metal saw blade, using a very special liquid to catch and prevent metal shavings from contaminating the station’s atmosphere – a dab of shaving gel from my personal hygiene kit. In my imagination I could see my childhood hero nodding at me with a smile. Almost exactly 4 years later, I returned to the International Space Station for my second mission, called “Horizons,” as the Commander of Expedition 57. One of the first things I did when arriving on ISS was to visit my favorite workplace in space, the ever-so-magnificent Columbus laboratory. And what I saw made me smile. I could see that the EML was still in its place, where I left it several years before, and it was still running flawlessly. It was operated very successfully and routinely by the international science team, conducting hundreds of experiment runs in the last years, for the benefit of humans on planet Earth. Hence, I am very excited about the results that the scientists have obtained on board the ISS and that are presented in this book. European Space Agency, Directorate of Human Robotic Exploration Programmes, LEO Exploration Group, ESTEC, Noordwijk, the Netherlands

Alexander Gerst

Foreword by Matthias Maurer

Future Metallurgy in Space The International Space Station is a unique laboratory that allows experiments not possible on Earth. In the fall of 2021, I will go to the International Space Station (ISS) for my mission “Cosmic Kiss.” Especially as a material scientist, I am excited to become directly involved with the materials science experiments in ESA’s Columbus module. For a long time, material scientists focused their investigations on the relation between mechanical and physical properties of solids related to their microstructures. Very soon, the importance of the defining steps for the microstructure, the processing route from the liquid melt to the final product, was recognized. Hence, in order to achieve new advancements in material science, a deeper understanding of the properties of the molten state is of key importance. Due to the high reactivity of most metallic melts, the investigation of liquid metals is best done by container-less methods. The weightlessness in the International Space Station allows the easy positioning of liquid metallic droplets. Furthermore, the absence of gravity allows precise measurements that are not possible on Earth. The obtained thermophysical properties of the investigated metallic alloys are used to answer basic scientific questions and are further utilized for the simulation of industrial production processes and the development of new materials. The upcoming experiments in the EML on board the International Space Station are concerned with materials that will improve life on Earth and also increase our abilities for space exploration. These experiments will improve steels for applications in energy conversion and electro-mobility, titanium alloys for 3D-printed biomedical implants, new metallic glasses for 3D printing of advanced structural, and functional components for several industries, including space flight and exploration.

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I am looking forward to the materials science experiments in the Columbus module, and I am very enthusiastic to support the continuation of the successful experiments that have been performed until now and are presented in this book. European Space Agency, Directorate of Human Robotic Exploration Programmes, LEO Exploration Group, EAC, Cologne, Germany

Matthias Maurer

Preface

Evolution of matter in the universe is one of the key elements related to our fundamental understanding of the formation of the planets and stars and life on Earth, and hence, has always evoked very keen interest in the scientific community. The natural forces existing on Earth, such as pressure, gravity, and strong or weak magnetic forces, are either absent or present in significantly different magnitude outside the Earth’s atmosphere. It is intuitive that formation of solids in outer space must have been through an entirely different environment and influence than what we can experiment with on Earth. Thus, experiments to determine thermophysical and thermochemical properties of common elements and compounds, of inorganic and organic in origin and nature, are essential to develop a fair understanding of the genesis and behavior of matter in outer space and simulate the same on Earth through rigorous modeling. This volume, entitled Metallurgy in Space, with selected chapters devoted to fundamental aspects of various intrinsic properties (melting/boiling point, viscosity, conductivity, diffusion coefficient, specific heat, and crystal structure), properties of pure solids (elements, alloys, and compounds), and response to external stimuli like temperature, pressure, and magnetic or electrical fields, both in reduced gravity condition in space and with usual gravitational environment on Earth, is absolutely essential. One of the highlights of this book arises from the articles dealing with experiments conducted in the International Space Station (ISS) and results obtained thereof concerning studies on various important thermophysical parameters of interest, which are not only unique but simply not known so far. These results and trends will greatly influence and impact our basic understanding in physical sciences as well as research and innovations in materials science and engineering in the future. A variety of industries – information technology, aerospace, automotive, biomedical, and basic and new materials manufacturing – need technological innovations, which attain high-value-added and high-quality products and at the same time environmental consciousness and regulations in a multibillion-dollar market. In recent years, the trend in developing new products moved from the traditional ix

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trial-and-error approach to computer-based modeling, for example, for hightemperature melt processing which represents almost 100% of all metal production technologies. This has become possible by the increase in computer power, but it is still hampered by a lack of available and appropriate liquid property data as reliable input parameters since – in a thermodynamic sense – “entropy wins at high temperature.” Recent progress in containerless levitation and processing techniques can overcome the experimental difficulties and enable measurements of various properties of “free-floating” metallic drops in the stable and undercooled liquid state in the hightemperature limit (in contrast to the famous Millikan experiment at Caltech about 100 years ago on oil drops at ambient conditions). With respect to this exciting development, truly international and multidisciplinary materials science projects (ThermoProp/ThermoLab – ISS) have been conceived over the last decades and supported by the various space agencies worldwide. Materials investigated include metallic alloys and composites, intermetallics, semiconductors, and glasses in the high-temperature limit. Basic metal physics aspects are considered as well, such as the atomic structure of complex multi-component liquids, their relation to macroscopic properties of the liquid phase, and the thermodynamics and kinetics of phase formation from the liquid which is of relevance for industrial alloy design. The measurements and investigations were performed and are still continuing onboard the International Space Station. Using the high-precision electromagnetic levitation device ISS-EML in the COLUMBUS module, the experimental temperature-time window available can be sufficiently extended to about 2200  C for more than 10,000 s. In this temperature-time regime, performance of controlled surface excitations, temperature modulation (A.C. calorimetry), and other techniques become reality in the high-temperature liquid state. The analysis based on high-precision video and temperature measurements as well as other sensing devices allows a in-depth study of a free-floating hot metallic liquid drop in equilibrium with different atmospheres for the first time ever. Besides basic scientific insight, this knowledge becomes also relevant for modern industrial processes such as high-precision casting, welding, 3D printing, energy conversion, and “green” processing. The experimental set-up is embedded in a truly international and world-renowned team of scientists in the field of thermophysics and the development of new materials in order to achieve the best science. Furthermore, the awareness and interest about the scope and need for such benchmark experiments allows to extend our deeper understanding of the origin and genesis of matter in space and evolution of properties of solids in space and on Earth. The future is wide open. Ulm, Germany Ranchi, Jharkhand, India

Hans-Jörg Fecht Indranil Manna

Contents

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hans-Jörg Fecht

Part I

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Programmatic, Facility and Infrastructure

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ESA’s Materials Science in Space Programme . . . . . . . . . . . . . . . . . Wim Sillekens

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The Electromagnetic Levitator Facility (EML) on Board the ISS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wolfgang Soellner and Winfried Aicher

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Operations of the Electromagnetic Levitator: From Spacelab to the ISS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rainer Willnecker and Angelika Diefenbach

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Electrostatic Levitation on the ISS . . . . . . . . . . . . . . . . . . . . . . . . . Takehiko Ishikawa and Paul-François Paradis

Part II

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Liquid Structure and Transition

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Atomic Structure in Metallic Liquids . . . . . . . . . . . . . . . . . . . . . . . Xiao-Dong Wang, Xue-lin Wang, Qing-Ping Cao, Dong-Xian Zhang, and Jian-Zhong Jiang

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Theory of Nucleation and Glass Formation . . . . . . . . . . . . . . . . . . . 153 Kenneth F. Kelton

Part III 8

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Ground Based Methods

Ground-Based Electromagnetic Levitation (EML) for the Measurement of Thermophysical Properties . . . . . . . . . . . . 181 Jürgen Brillo xi

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The Measurement of Density, Surface Tension, and Viscosity of Metallic Liquids by the Discharge Crucible Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Quentin Champdoizeau and Hani Henein

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An Overview of Ground-Based Electrostatic Levitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Michael P. SanSoucie

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Levitation Research in Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 Masahito Watanabe, Shumpei Ozawa, Hiroyuki Fukuyama, Takao Tsukada, and Taketoshi Hibiya

Part IV

Thermophysical Property Measurement by Levitation

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Measurement of Thermophysical Properties Using the ISS-EML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 Markus Mohr and Hans-Jörg Fecht

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Modeling of Magnetohydrodynamic Flows in Electromagnetic Levitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Gwendolyn P. Bracker and Robert W. Hyers

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Influence of Convection on Phase Selection . . . . . . . . . . . . . . . . . . . 299 Douglas M. Matson

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Thermophysical Properties of Ni-Based Superalloys . . . . . . . . . . . . 315 Rada Novakovic, Donatella Giuranno, Markus Mohr, Jürgen Brillo, and Hans-Jörg Fecht

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Thermophysical Properties of Titanium Alloys . . . . . . . . . . . . . . . . 357 Markus Mohr, Rainer Wunderlich, and Hans-Jörg Fecht

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Thermophysical Properties of Steels . . . . . . . . . . . . . . . . . . . . . . . . 377 Seshadri Seetharaman, Livio Battezzati, Markus Mohr, and Hans-Jörg Fecht

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Thermophysical Properties of Semiconductors . . . . . . . . . . . . . . . . 403 Yuansu Luo, Bernd Damaschke, Georg Lohöfer, and Konrad Samwer

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Thermophysical Properties of Bulk Metallic Glasses . . . . . . . . . . . . 425 Markus Mohr, Yue Dong, Douglas C. Hofmann, Antonia Neels, Alex Dommann, William L. Johnson, and Hans-Jörg Fecht

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Electrical Resistivity Measurements on the International Space Station for the Studies of Dynamics in Metallic Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 Anup K. Gangopadhyay and Kenneth F. Kelton

Contents

Part V

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Technology Trends and Future Perspectives

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New Material Developments/High-Entropy Alloys . . . . . . . . . . . . . 473 Yannick Champion

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Laser-Assisted Additive Manufacturing of Ni-Based Superalloy Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 Manoj Kumar, Jyotsna Dutta Majumdar, Hans-Jörg Fecht, and Indranil Manna

Annex: Related Publications of the Last Decade . . . . . . . . . . . . . . . . . . . 523 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549

Contributors

Winfried Aicher Airbus Defence and Space, Friedrichshafen, Germany Livio Battezzati Dipartimento di Chimica, Università di Torino, Torino, Italy Gwendolyn P. Bracker Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA, USA Jürgen Brillo Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Köln, Germany Qing-Ping Cao International Center for New-Structured Materials (ICNSM), State Key Laboratory of Silicon Materials, and School of Materials Science and Engineering, Zhejiang University, Hangzhou, People’s Republic of China Quentin Champdoizeau Faculty of Engineering – Chemical and Materials Engineering Department, University of Alberta, Edmonton, AB, Canada Yannick Champion CNRS, Grenoble INP, SIMaP, University of Grenoble Alpes, Grenoble, France Bernd Damaschke I. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany Angelika Diefenbach Microgravity User Support Center, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Köln, Germany Alex Dommann Center for X-Ray Analytics, Empa Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland Hans-Jörg Fecht Institute of Functional Nanosystems, Ulm University, Ulm, Germany Hiroyuki Fukuyama Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai, Japan Anup K. Gangopadhyay Washington University in St. Louis, St. Louis, MO, USA xv

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Alexander Gerst European Space Agency (ESA), Directorate of Human Robotic Exploration Programmes, LEO Exploration Group, ESTEC, Noordwijk, the Netherlands Donatella Giuranno Institute of Condensed Matter Chemistry and Technologies for Energy, National Research Council (CNR-ICMATE), Genoa, Italy Hani Henein Faculty of Engineering – Chemical and Materials Engineering Department, University of Alberta, Edmonton, AB, Canada Taketoshi Hibiya Graduate School of System Design and Management, Keio University, Yokohama, Kanagawa, Japan Douglas C. Hofmann Materials Development and Manufacturing Technology Group, NASA Jet Propulsion Laboratory/California Institute of Technology, Pasadena, CA, USA Robert W. Hyers Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA, USA Takehiko Ishikawa Japan Aerospace Exploration Agency, Tsukuba, Ibaraki, Japan Jian-Zhong Jiang International Center for New-Structured Materials (ICNSM), State Key Laboratory of Silicon Materials, and School of Materials Science and Engineering, Zhejiang University, Hangzhou, People’s Republic of China William L. Johnson Keck Engineering Laboratories, California Institute of Technology, Pasadena, CA, USA Kenneth F. Kelton Washington University in St. Louis, St. Louis, MO, USA Manoj Kumar Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India CSIR – Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India Georg Lohöfer Institut för Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Köln, Germany Yuansu Luo I. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany Jyotsna Dutta Majumdar Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Indranil Manna Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Birla Institute of Technology, Mesra, Ranchi, Jharkhand, India Douglas M. Matson Department of Mechanical Engineering, Tufts University, Medford, MA, USA

Contributors

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Matthias Maurer European Space Agency (ESA), Directorate of Human Robotic Exploration Programmes, LEO Exploration Group, EAC, Cologne, Germany Markus Mohr Institute of Functional Nanosystems, Ulm University, Ulm, Germany Antonia Neels Center for X-Ray Analytics, Empa Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland Rada Novakovic Institute of Condensed Matter Chemistry and Technologies for Energy, National Research Council (CNR-ICMATE), Genoa, Italy Shumpei Ozawa Department of Advanced Materials Science and Engineering, Chiba Institute of Technology, Narashino, Chiba, Japan Paul-François Paradis I.N.O., Quebec-city, QC, Canada Konrad Samwer I. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany Michael P. SanSoucie Materials and Processes Laboratory, Metallic Materials & Processes Division (EM30), NASA Marshall Space Flight Center (MSFC), Huntsville, AL, USA Seshadri Seetharaman Royal Institute of Technology, Stockholm, Sweden Wim Sillekens European Space Agency – ESTEC, Noordwijk, the Netherlands Wolfgang Soellner Airbus Defence and Space, Friedrichshafen, Germany Takao Tsukada Department of Chemical Engineering, Tohoku University, Sendai, Japan Xiao-Dong Wang International Center for New-Structured Materials (ICNSM), State Key Laboratory of Silicon Materials, and School of Materials Science and Engineering, Zhejiang University, Hangzhou, People’s Republic of China Masahito Watanabe Department of Physics, Gakushuin University, Toshima, Tokyo, Japan Rainer Willnecker Microgravity User Support Center, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Köln, Germany Rainer Wunderlich Institute of Functional Nanosystems, Ulm University, Ulm, Germany Xue-lin Wang International Center for New-Structured Materials (ICNSM), State Key Laboratory of Silicon Materials, and School of Materials Science and Engineering, Zhejiang University, Hangzhou, People’s Republic of China Dong-Xian Zhang International Center for New-Structured Materials (ICNSM), State Key Laboratory of Silicon Materials, and School of Materials Science and Engineering, Zhejiang University, Hangzhou, People’s Republic of China

Chapter 1

Introduction Hans-Jörg Fecht

1 General Material scientists originally devoted most of their efforts to studying the solid state of materials, their microstructure, and their mechanical and thermal properties. However, in the last 10–20 years, a change in paradigm has taken place, and the importance of the liquid phase has been recognized. In this regard, it is interesting to note that almost 100% of all metallic products are, at some stage, produced through solidification and casting processes. Consequently, this field of new materials, processes, and products constitutes a major backbone to industries worldwide. Solidification from the melt leaves its fingerprints in the final material, and hence it is of utmost importance to understand the properties of the molten state and its solidification behavior. The prominent feature of fluids, namely, their ability to flow and to form free surfaces, poses the main difficulty in their theoretical description. The physics of fluids is governed by the Navier-Stokes equation and by the ubiquitous presence of convection. In addition, when dealing with metallic materials, the high temperatures involved lead to experimental difficulties, the most trivial but also most fundamental being the suitability of available containers. Besides the atomic scale inherent to condensed matter and the intermediate scales associated with the solidification microstructures, fluid flow driven by gravity generally occurs in the melt at the macroscopic level so that the relevant length scales in casting are widespread from the atomic size (capillary length, crystalline defects such as dislocations, attachment of atoms, etc.) to the meter size of the ingot (fluid flow, spacing of dendrite side branches, etc.). Accordingly, to produce materials that meet ever-higher specific requirements and performance, the solidification processing of structural and functional materials

H.-J. Fecht (*) Institute of Functional Nanosystems, Ulm University, Ulm, Germany e-mail: [email protected] © The Minerals, Metals & Materials Society 2022 H.-J. Fecht, M. Mohr (eds.), Metallurgy in Space, The Minerals, Metals & Materials Series, https://doi.org/10.1007/978-3-030-89784-0_1

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has to be controlled with ever-increasing precision. It can be foreseen that materials for tomorrow will be optimized in their design and underlie more efficient production conditions, availability of scarce resources, and cleaner processes. The interactive feedback between experiments and sophisticated computer simulations developed within the last 10 years that now drives the design and processing of materials is reaching performances never been seen in the past. Thus, it becomes possible to control and optimize the defect and grain structure at critical patches of components. Here, two major aspects are most essential for the continued improvement of materials processing with increasing requirements on composition, microstructure, and service achievements, which often implies the breaking of technology barriers: • The reliable determination of the thermophysical properties of metallic melts in order to understand the fundamentals of complex melts and their influence on the nucleation of ordered phases. • The reliable determination of the formation and selection mechanisms at microstructure scales in order to understand the fundamentals of casting and other solidification processes (foundry, welding, brazing, atomization,. . .). This also requires accurate knowledge of thermophysical properties.

2 Scientific Challenges Casting is a non-equilibrium process by which a liquid alloy is solidified. The liquidsolid transition is driven by the departure from thermodynamic equilibrium, where no change can occur. From the standpoint of physics, casting thus belongs to the vast realm of out-of-equilibrium systems, which means that, rather than growing evenly in space and smoothly in time, the solid phase prefers to form a diversity of microstructures. Actually, the relevant length scales in casting are widespread over ten orders of magnitude. At the nanometer scale, the atomic processes determine the growth kinetics and the solid-liquid interfacial energy, and crystalline defects such as dislocations, grain boundaries, and voids are generally observed. Macroscopic fluid flow driven by gravity or imposed by a stimulus (electromagnetic field, vibration, etc.) occurs in the melt at the meter scale of the cast product. The characteristic scales associated with the solidification microstructures are mesoscopic, i.e., intermediate, ranging from dendrite tip/arm scale (1–100 μm) to the grain size (mm–cm). It follows that the optimization of the grain structure of the product and inner microstructure of the grain(s) during the liquid-to-solid phase transition is paramount for the quality and reliability of castings, as well as for the tailoring of new advanced materials for specific technological applications. On this basis, the quantitative numerical simulation of casting and solidification processes is increasingly demanded by manufacturers, compared to the wellestablished but time-consuming and costly trial-and-error procedure. It provides a

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Fig. 1.1 A wide range of fundamental events during casting of complex components – here, a car engine with varying local temperatures. (Courtesy MagmaSoft)

rapid tool for the microstructural optimization of high-quality castings, in particular where process reliability and high geometric shape accuracy are important (see, e.g., Fig. 1.1 exhibiting cast structural components and the temperature distribution during casting of a car engine block). Any improvement of numerical simulation results in improved control of fluid flow and cooling conditions that enable further optimization of the defect and grain structure as well as stress distribution at critical patches of components. Through the control of unwanted crystallization events, it becomes even possible to produce completely new materials with a controlled amorphous (glassy) or nanocomposite structure.

3 Microgravity Space Conditions and Containerless Processing The paucity of thermophysical property data for commercial materials as well as materials of fundamental interest is a result of the experimental difficulties arising at high temperatures. Some of these data can be obtained more or less accurately by conventional methods, in particular for non-reactive metals such as noble metals. High-precision measurements, however, on chemically highly reactive melts at the temperatures of interest require the application of containerless processing techniques and the use of high-precision non-contact diagnostic tools. By eliminating the contact between the melt and a crucible, accurate surface nucleation control and the synthesis of materials free of surface contamination become possible. For highly reactive metallic melts, electromagnetic levitation

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Fig. 1.2 Electromagnetic processing on the ground (left) and in microgravity (right) – the latter allowing controlled investigations of fully spherical liquid metallic samples (6.5–8 mm diameter) with a wide range of sophisticated analytical equipment. (Courtesy DLR)

(EML) is a well-developed containerless technique that offers several advantages over alternative levitation methods (electrostatic levitation, gas-phase levitation) due to the direct coupling of the high-intensity radiofrequency electromagnetic field with the sample. Ground-based experiments using electromagnetic levitation have achieved limited success in measuring thermophysical properties of liquid alloys, since the high electromagnetic field B required to lift the sample against gravity (Lorentz force F / ∇ B2) also causes excessive heating and turbulence due to induced eddy currents. In contrast, under microgravity conditions, much smaller levitation forces are needed since the force of gravity no longer has to be overcome. In fact, in space, only a weak positioning field is required. This means that heating effects, magnetic pressure, melt turbulence, and asphericity of the molten drop are significantly reduced, allowing considerably more accurate results to be obtained or making such experiments possible at all. As an example, Fig. 1.2 shows a comparison between a specimen levitated in a ground-based em-levitation (left) and a liquid specimen positioned under reduced gravity conditions in an em-levitation device on board a parabolic flight (right). As compared to the specimen levitated on the ground, the specimen positioned under reduced gravity exhibits no detectable deviation from a spherical shape. The motivation for performing benchmark experiments in the microgravity environment thus is straightforward and at a high level of scientific innovation. Firstly, in space it is possible to suppress the gravity-induced effects of fluid flow and more subtle sedimentation effects during solidification. Therefore, the contribution to fluid flow and heat transport in the melt can be investigated without the complications of buoyancy-driven thermo-solutal convection and sedimentation/flotation.

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Secondly, the space environment on long time scales allows the application of containerless processing techniques, such as electromagnetic levitation. Levitated melts can be controlled effectively at temperatures up to 2200  C, which in turn enables critical liquid parameters to be measured much more accurately and in a larger temperature range as compared to the earth laboratory. Experience with parabolic flights (μg duration 10–20 s) and TEXUS rocket flights (μg duration ca. 180 s) already indicated that some aspects of the experiments could be successfully performed, but μg times are far too short to reach thermal equilibrium and measurements in the adiabatic regime. Expanding the experimental timetemperature window through the use of the International Space Station (ISS) opens a completely new realm of space experimentation. The main advantages in this regard can be summarized as follows: • Avoidance of any chemical reactions with a metallic or ceramic container • Decoupling of electromagnetic heating and positioning fields, therefore minimized levitation forces and, thus, controlled heating and reduced liquid convection in comparison with 1-g gravity conditions on earth • Achievement of fully spherical samples • Control of the sample environment (and cooling rate) in vacuum (better than 108 Torr) or inert gas atmosphere • Extended periods of processing time (>10,000 s) in a temperature range between 700 and 2200  C. • Considerably improved accuracy of the measurements.

4 Experimental Program The processing of metallic alloys (a combination of two or more elemental metals) through melting and casting techniques, whereby the molten material is poured or forced into a mold and allowed to harden, was invented several thousand years ago. Today, this processing is still an important step in the industrial production chain for a wide range of products. The end products often need to perform well and retain their integrity under extreme circumstances, particularly when used at high temperatures or when the product must be as light as possible in order to conserve energy. To produce these high-performance materials, the process must be closely controlled for the sake of both optimal design and efficiency of production. The production and fabrication of alloys together with the casting and foundry industry generate a considerable amount of wealth. For example, the 10 million tons of castings produced in 1 year within the European Union is worth about 20 billion Euros. To continue generating this kind of turnover, the casting and foundry industry relies on the design and creation of advanced materials, which is accomplished by using sophisticated computer codes to control the metallurgical processes. These days everyone is looking for the next great breakthrough that leaps forward in technology that revolutionizes the way business is done. The answer may lie in a

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surprising place: space. In the last years, a scientific program has been established by the European Space Agency (ESA) using weightlessness as an important research tool on parabolic flights, on sounding rockets, and, most recently, on the International Space Station. Experiments performed in microgravity enable the study of the relevant volume and surface-dependent properties free of certain restrictions of a gravity-based environment. In space it is possible to suppress gravity’s effects on the flow of molten metals and on sedimentation during solidification. Without gravity’s interference, it is possible to isolate other properties for investigation, such as diffusion and how it contributes to mass and heat transport in the melt without the gravityassociated complications of certain solute ingredients being more buoyant than others. Using advanced experimental techniques to gather data on the intricate processes of melting and casting brings us closer to the design of new materials with better performance. Such advanced products can range from meter-sized objects to micrometer-sized powders, for example: • Energy-efficient turbine components for the aerospace industry and land-based power plants. • Powder production to improve catalytic performance of modern fuel cells and advanced combustion engines. • High-strength metals with added functionalities. • Precision casting of detailed shapes for electronic casings. • Low-weight and high-strength materials for modern space vehicles within the space exploration programs. • Medical implants. In order to perform the necessary experiments, it is important to have access to extended periods of reduced gravity. A crucial ISS facility is the electromagnetic levitator (EML). As fantastic as it sounds, this equipment does precisely what the name implies: levitated molten metals. The EML permits containerless melting and solidification of alloy samples. Furthermore, the EML is equipped with highly advanced diagnostic tools that permit accurate measurements of thermophysical properties, as well as direct observation of the experiment during flight by highspeed videography. As the products we make become more sophisticated, it follows that their production processes must keep up. Advancements in liquid processing techniques have enabled the industry to create products such as jet engines, spacecraft, and medical implants, but society’s push for continually stronger, lighter, and more efficient products requires that next great leap.

Part I

Programmatic, Facility and Infrastructure

Chapter 2

ESA’s Materials Science in Space Programme Wim Sillekens

1 Introduction Within ESA, its member and cooperating states are working together on space research and technology and their space applications. Activities relating to the exploration destinations Low-Earth Orbit, Moon and Mars are integrated into a single European Exploration Programme (E3P), of which the “Science in Space Environment” (SciSpacE) element is concerned with the scientific research on the ISS, non-ISS space platforms and space-environment analogues. SciSpacE and its preceding “European Programme for Life and Physical Sciences in Space” (ELIPS) are and have been providing the scientific communities in the relevant disciplines with experiment opportunities using these platforms since the start of ISS utilisation at the turn of the century and as an extension of the initial European microgravity programmes going back to the 1980s. Descriptions and results of these experiments are archived in a publicly accessible and searchable ESA repository [1]. The overall motivation for conducting science in a space environment is that this reveals features of terrestrial life and physical processes that cannot be observed and/or controlled on Earth. Aspects of interest include – but are not limited to – the reduced-gravity condition, the otherwise extreme conditions (in their possibly wide sense, ranging from radiation and temperature variations to remoteness and confinement) and the vantage point for Earth as well as for deep space. Research activities that are being developed and conducted in this context are correspondingly diverse. These research activities are being guided by the so-called science roadmaps (or research agendas) that have been established by the European scientific communities for the respective domains and are documented in [2] – with an updated second issue that now also includes the Moon and Mars destinations to be

W. Sillekens (*) European Space Agency – ESTEC, Noordwijk, the Netherlands e-mail: [email protected] © The Minerals, Metals & Materials Society 2022 H.-J. Fecht, M. Mohr (eds.), Metallurgy in Space, The Minerals, Metals & Materials Series, https://doi.org/10.1007/978-3-030-89784-0_2

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Fig. 2.1 ESA’s top-level roadmap themes for Low-Earth Orbit research activities

published in 2021. Roadmap themes are structured as shown in Fig. 2.1, with materials science being covered in the physical sciences box under the title Advanced Material Processing. Here, the implication of a reduced-gravity environment (typically 600  C) 150 μΩ  cm show a negative value of dρ/dT [59], although there are exceptions [60]. One of the most influential theories for metallic liquids was developed by Ziman and coworkers [61, 62]. The resistivity was expressed in terms of an integral that contains the structure factor, S(q) (partial S(q)s for alloys), and the pseudopotential, |V(q)| [61, 62], or the scattering matrix (t-matrix) [63, 64]. In both cases, the sign of dρ/dT arises due to the temperature dependence of the first peak height of S(q) and its position with respect to the Fermi wave vector, kf. When 2kf lies on the higher
q-side  of the first peak of S(q), dρ/dT becomes negative because of the decrease of S q2kF with increasing temperature. Since the higher q-side of the peak of S(q, T ) makes a much bigger contribution to this integral, the sign of dρ/dT is very sensitive to the position of 2kf with respect to the peak position of S(q, T ). Electrical resistivity measurements for metallic glasses have been mostly confined to near and below Tg [47, 48, 57, 58]. Fewer studies have been reported for the corresponding liquids; some examples are provided in refs. [65–69]. The temperature dependence of the resistivity may be only qualitatively explained by FaberZiman-type theories because of difficulties in estimating 2kf. Reliable experimental data for more metallic liquids at high temperatures are needed to develop more quantitative models. This was one motivation for the experiments that are described here. A second motivation is that since the resistivity is determined by the structure factor, and since electrons scatter more strongly from the structure than to x-rays or neutrons, the electrical resistivity might be sufficiently sensitive to detect the subtle structural changes at TA. This was found to be the case.

3 Electrical Resistivity Measurement The four-probe technique (with two current and two voltage leads connected to a sample) is the standard method for measuring the electrical resistivity of solids and liquids. The experimental results for the glasses at low temperatures, which will be

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presented later, were obtained by using this method. However, chemical reaction with the current and voltage leads often cause problems for the high Tl liquids. These may be eliminated by contactless methods, such as mutual inductance-type ac measurements [70, 71]. This consists of two coils, a primary one to generate a small ac magnetic field (a few Gauss) and a secondary coil wound on top of the primary near its center to pick up a signal from the sample. The ac magnetic field generates eddy currents on the surface of the sample, which interact with the applied field and change the mutual inductance between the coils. The mutual inductance is a complex quantity; the real part is related to the magnetic moment (susceptibility) and the imaginary part (dissipative loss due to electrical resistance) to the electrical resistivity. Such simple homebuilt devices are used in many laboratories around the world; commercial products are also available. Although this technique eliminates the need for attaching electrical leads, containers are still required. To the best of our knowledge, the first noncontact containerless resistivity measurements on levitated Si and Ge liquids were made using an aerodynamic levitator [72]. The change in the inductance of a coil inside the chamber was measured when a levitated sample was inserted into it. However, as mentioned in §2.1, the aerodynamic levitation technique is not suitable for measurements on metallic liquids, especially in the supercooled state. The ESL was used to measure the electrical resistivity of liquid Al by Rhim et al. [73] using a different measurement technique. The electrical resistivity was determined from the torque experienced by a liquid sphere of Al under vacuum in a rotating magnetic field, which was produced by passing ac currents in four identical coils mounted on the positioning electrodes. This assembly acted like an asynchronous induction motor, where the coils acted as the stator and the sample as a rotor. The ISS-EML facility uses the same principle as used in mutual inductance measurements but with a very different coil geometry [21]. Here, only the most essential aspects of this technique are discussed. As shown in Fig. 20.6, the EML consists of a pair of identical water-cooled Cu coils. Two separate circuits feed ac voltages into these coils. One is a positioner voltage from a power amplifier that feeds equal amounts of currents in the opposite directions into the upper and lower coils at 150 kHz. This produces a quadrupolar radio frequency magnetic field. A spherical, 6–8-mm-diameter sample is positioned by this field at the center of the coils where the field is minimum. Because of the microgravity environment, no force is necessary to levitate the sample; the quadrupolar field just maintains the sample in position. A separate power amplifier feeds currents in the same directions into the two coils at about 350–400 kHz to produce a dipolar field for heating the sample. Therefore, heating and positioning can be controlled independently, which is different from most of the ground-based EML instruments. The total complex admittance of the electrical heating circuit is given by [21, 74]:

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Fig. 20.6 The electrical circuit for resistivity measurements on the ISS-EML. Details are provided in the text. (Reprinted with permission from Ref. [20]. Copyright (2019) by the American Physical Society)

e tot ¼ 2iωC þ Y

2 , eS ða, ρÞ=2 RL þ iωL þ Z

ð20:2Þ

where C is the capacitance of the condenser, L is the inductance, and RL is the e S ða, ρÞ is the complex impedance of the sample, which resistance of the coil; Z depends on the sample radius, a, and its electrical resistivity, ρ. The sample coupling electronics (SCE) unit in the EML facility on the ISS measures the amplitude of the RF current through the circuit, Io, the voltage drop over the circuit, Uo, and the phase shift, φ, between the voltage and current and the frequency, ω. The total admittance of the circuit can then be obtained as e tot ¼ I 0 eiφ , Y Uo

ð20:3Þ

e S ¼ 0 , the circuit parameters C, L, and RL can be Without a sample, i.e., for Z e tot with obtained from Eq. (20.2). With these data, a subsequent measurement of Y e a sample in the levitator yields the sample impedance, Z S ða, ρÞ . For a spherical sample (as is the case for the liquid under microgravity) in a homogeneous RF eS ða, ρÞ, the sample radius, a, and magnetic field, the theoretical relation between Z the resistivity, ρ, has been calculated in refs. [21, 74]. The sample is first melted and heated to about 300–400 K above the melting temperature under high vacuum (108Torr) or in inert gas atmosphere and then cooled with very small heater (0.1 V) and positioner (2–3 V) voltages so that shape distortions of the liquid droplet remain minimal. This allows the temperature-dependent sample radii to be determined from the video images of the levitated droplet using standard procedures [21, 75], which are required to estimate the electrical resistivity from the measured sample impedance.

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4 Electrical Resistivity of Cu50Zr50 and Zr64Ni36 Liquids Figure 20.7a, b show the measured electrical resistivity as a function of temperature for two alloys, Zr64Ni36 and Cu50Zr50, respectively, in the equilibrium and supercooled states. The former is a marginal glass former (critical thickness less than 100 microns) [76], and the latter forms a bulk metallic glass [77]. The TL for these alloys are 1283 and 1222 K, respectively. The precision in the resistivity measurement is about 2%, 1% of which comes from the sample radius measurement. However, relative changes can be measured to approximately a 0.7% precision. The raw data are shown in the insets, and the 200-point adjacent smoothed data are shown in the main figures. Both alloys show negative temperature coefficients of resistivity, dρ/dT; this is expected [59] since the magnitudes of the resistivities are large. This can be explained qualitatively by modified Ziman theory [62–64], using partial structure factors obtained from MD simulations (experimental S(q, T ) partials would need to be determined from neutron scattering experiments using isotopically enriched samples) and reasonable values of the Fermi wave vector (kf) [supplemental in 20, 78]. The most interesting observation, however, is the saturation/near saturation of resistivity above TA (shown by the shaded regions). This cannot be explained by the Ziman theory since the first peak of the total S(q, T ) and the partial structure factors continue to decrease above TA, indicating that there should be a continuous decrease in resistivity, not a saturation. An alternative explanation to the Faber-Ziman-type theories for the negative dρ/ dT in glasses at low temperature was suggested a few decades ago [79, 80]. As mentioned in §2.4, electrons become localized in the presence of strong disorder,

Fig. 20.7 The electrical resistivities of Zr64Ni36 (a) and Cu50Zr50 liquids. Both show negative temperature coefficients and near saturation above TA (shaded regions). TA was determined from viscosity measurements (Fig. 20.1). (The data are reprinted with permission from Ref. [20]. Copyright (2019) by the American Physical Society)

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Fig. 20.8 The electrical conductivities of Zr64Ni36 (a) and Cu50Zr50 increases pffiffiffiffi as T for the glasses. The liquid data are the same as in Fig. 20.7. TA and the glass transition temperature, Tg, are marked by dashed vertical lines. (The data are reprinted with permission from Ref. [20]. Copyright (2019) by the American Physical Society)

which is called Anderson localization. A similar concept of “weak localization” due to chemical and structural disorder was introduced for the metallic glasses at low temperatures [79, 80]. However, this differs from Anderson localization, where the resistivity increases following a T1/4 law [81] as the electrons start to hop from one atomic site to another in an activated process. In the weak localization theories, multiple inelastic scattering of electrons by phonons are coherent at low temperatures. As the temperature increases, the loss of coherence in the scattering weakens electron localization, and the electrical conductivity, σ(T), pffiffiffiffi increases (the resistivity decreases) almost linearly at low temperatures and as T above the Debye temperature, ΘD [80]. This behavior has been observed at low temperatures in the Cu50Zr50 glass [80]. Figure 20.8 shows the electrical conductivity, σ(T), for the two glasses at low temperature, pffiffiffiffi along with those of the liquids at high temperature. The data seem to follow a T dependence for both glasses; however, because of the limited temperature range, its functional relationship with temperature in the liquid is not clear. Since no theory exists that predicts a saturation of the resistivity/conductivity in liquids, a qualitative argument is provided [20]. Tg is usually defined as the temperature where the structural relaxation time is about 100 s. As the temperature increases, the relaxation time decreases sharply, in a similar fashion as the viscosity. Since the electron scattering times are in the nano- to femtosecond ranges, such structural changes in supercooled liquids above Tg appear static to the electrons. Therefore, the increase in electrical conductivity above and below Tg should not be entirely different. However, at higher temperatures, the short- and medium-range order also starts to change rapidly. Since liquids cannot sustain shear, the normal concept of phonons breaks down in liquids. However, the high-frequency (instantaneous) shear modulus remains finite, as was discussed in §2.2. Two things might happen in the liquid with increasing temperature. First, the structural relaxation time

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may become comparable to the electron scattering mean free time at higher temperatures. This may add an additional scattering mechanism for the electrons. If the structural relaxation time decreases faster than the electron scattering time, at some point again, the electrons cannot follow the structural changes, and this channel of scattering will become temperature independent. Second, as discussed in §2.2, the ratio of the Maxwell relaxation time, τM (which governs the structural relaxation time), and the time for adding/removing an atom from a cluster, τLC, becomes equal and temperature independent above TA. Since such events of structural changes happen fast, this information cannot be conveyed to the nearest cluster. This inability to communicate is basically equivalent to the localization of the high-frequency phonons. As a result, electron scattering by the high-frequency phonons will also become temperature independent. Both of these mechanisms may contribute to a near saturation of the electron mean free path and, therefore, a saturation of the electrical conductivity/resistivity. Importantly, since this happens at TA, these data provide the first direct experimental evidence for the connection between the local structure of the liquid and the liquid dynamics, which was predicted by the earlier MD simulations.

5 Conclusions The work presented here demonstrates that the structural dynamics can be probed by electrical resistivity in equilibrium and supercooled liquids. It opens the possibility of using this technique to also probe other structural changes associated with liquidliquid transitions, chemical clustering, phase separation, etc. The saturation of the resistivity is a novel observation that could only be obtained using the SCE unit in ISS-EML. Although demonstrated only for two alloy liquids, similar behavior was observed in Ti39.3Zr39.5Ni21 and indicated in Vit106 (Zr57Cu15.4Ni12.6Al10Nb5) liquids. However, since those liquids were processed in metallic cage sample holders on the ISS-EML, the results are somewhat uncertain; interaction of the metallic cage with the rf-field may have influenced those measurements. We are aware of only one other case where an indication of resistivity saturation in a high-temperature liquid was reported [69]. However, the reason for this was chemical phase separation in immiscible Cu-Co liquids. With large negative heats of mixing for Zr64Ni36 (57 kJ/ mol) and Cu50Zr50 (34 kJ/mol) [82], this possibility can be ruled out for the present cases. Future experiments are planned on many more liquids using the ISS-EML facility to verify whether this is a common phenomenon in all liquids. If our postulate that this is associated with the crossover temperature is correct, it should be observed in almost all liquids. Acknowledgments Work at Washington University was supported by NASA under grant Nos. NNX10AU19G and NNX16AB52G. The authors acknowledge the access to the ISS-EML, which is a joint undertaking of the European Space Agency (ESA) and the DLR space administration. We

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are particularly indebted to the members of the Microgravity Users Support Center (MUSC) at the DLR, Köln, for their generous support in the planning and execution of the experiments.

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structure, volume, and cohesive energy. J. Chem. Phys. 146, 154506 (2017). https://doi.org/10. 1063/1.4981011 40. R. Dai, R. Ashcraft, K.F. Kelton, A possible structural signature of the onset of cooperativity in metallic liquids. J. Chem. Phys. 148, 204502 (2018). https://doi.org/10.1063/1.5026801 41. S.S. Schoenholz, E.D. Cubuk, D.M. Sussman, E. Kaxiras, A.J. Liu, A structural approach to relaxation in glassy liquids. Nat. Phys. 12, 469–472 (2016). https://doi.org/10.1038/ NPHYS3644 42. J. Ding, Y.-Q. Cheng, H. Sheng, M. Asta, R.O. Ritchie, E. Ma, Universal structural parameter to quantitatively predict metallic glass properties. Nat. Commun. 7, 13733 (2016). https://doi.org/ 10.1038/ncomms13733 43. N.W. Ashcroft, N.D. Mermin, Solid State Physics (Saunders College Publishing, New York, 1976) ISBN 0-03-083993-9. OCLC 934604 44. S. Sachdev, Quantum Phase Transitions (Cambridge University Press, New York, 1999) 45. A.W. McReynolds, Electrical observations of the austenite-martensite transformation in steel. J. Appl. Phys. 17, 823 (1946). https://doi.org/10.1063/1.1707649 46. L.W. Shacklette, W.S. Williams, Influence of order-disorder transformations on the electrical resistivity of vanadium carbide. Phys. Rev. B 7, 5041–5053 (1973). https://doi.org/10.1103/ PhysRevB.7.5041 47. K.F. Kelton, F. Spaepen, Kinetics of structural relaxation in several metallic glasses observed by changes in electrical resistivity. Phys. Rev. B 30, 5516–5524 (1984). https://doi.org/10.1103/ PhysRevB.30.5516 48. A.K. Gangopadhyay, T.K. Croat, K.F. Kelton, The effect of phase separation on subsequent crystallization in Al88Gd6La2Ni4. Acta Metall. 48, 4035–4043 (2000). https://doi.org/10. 1016/S1359-6454(00)00196-8 49. P.W. Anderson, Absence of diffusion in certain random lattices. Phys. Rev. 109, 1492–1505 (1958). https://doi.org/10.1103/PhysRev.109.1492 50. N.F. Mott, Metal-Insulator Transitions (Taylor and Francis, London, 1974), p. 23 51. P.A. Lee, T.V. Ramakrishnan, Disordered electronic systems. Rev. Mod. Phys. 57, 287–337 (1985). https://doi.org/10.1103/RevModPhys.57.287 52. O. Gunnarsson, M. Calandra, J.E. Han, saturation of electrical resistivity. Rev. Mod. Phys. 75, 1085–1099 (2003). https://doi.org/10.1103/RevModPhys.75.1085 53. R.A. Davison, K. Schalm, J. Zaanen, Holographic duality and the resistivity of strange metals. Phys. Rev. B 89, 245116 (2014). https://doi.org/10.1103/PhysRevB.89.245116 54. W. Klement, R.H. Willens, P. Duwez, Non-crystalline structure in solidified gold–silicon alloys. Nature 187, 869–870 (1960). https://doi.org/10.1038/187869b0 55. A.J. Drehman, A.L. Greer, D. Turnbull, Bulk formation of a metallic glass. Appl. Phys. Lett. 41, 716–717 (1982). https://doi.org/10.1063/1.93645 56. U. Mizutani, Electronic structure of metallic glasses. Prog. Mater. Sci. 28, 97–228 (1983). https://doi.org/10.1016/0079-6425(83)90001-4 57. M.A. Howson, B.L. Gallagher, The electron transport properties of metallic glasses. Phys. Rep. 170, 265–324 (1988). https://doi.org/10.1016/0370-1573(88)90145-7 58. U. Mizutani, Electron transport in non-periodic metallic systems: Amorphous alloys and quasicrystals. Phys. Status Solidi B 176, 9–30 (1993). https://doi.org/10.1002/pssb. 2221760102 59. J.H. Mooij, Electrical conduction in concentrated disordered transition metal alloys. Phys Status Solidi A 17, 521–530 (1973). https://doi.org/10.1002/pssa.2210170217 60. C.C. Tsuei, Nonuniversality of the Mooij correlation the temperature coefficient of electrical resistivity of disordered metals. Phys. Rev. Lett. 57, 1943–1946 (1986). https://doi.org/10.1103/ PhysRevLett.57.1943 61. J.M. Ziman, A theory of electrical properties of liquid metals: I The monovalent metals. Philos. Mag. 6, 1013–1035 (1961) 62. T.E. Faber, Introduction to the Theory of Liquid Metals (Cambridge University Press, 1972)

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Part V

Technology Trends and Future Perspectives

Chapter 21

New Material Developments/High-Entropy Alloys Yannick Champion

1 Introduction This chapter is dedicated to the development of new materials, more specifically new metallic alloys called high-entropy alloys (HEA) [1]. A very first definition is that HEA are solid solutions containing at least five metallic elements in equi-atomic composition. We shall see that HEAs will deviate from this strict definition and eventually extend to complex concentrated alloys (CCAs) which have higher degrees of flexibility in chemical composition and microstructure. The chapter reports on the main characteristics and properties of a new class of alloys, discovered in 2004, then having no applications so far but many perspectives. Beyond the objective of being informative, obvious perspectives for these alloys are crucially related to their development, and the importance of investigations and measurements such those performed in space and parabolic flights are of course no longer to be demonstrated. With the word “new” in the title of this chapter, there is a strong temptation to start with a frequent question, which is whether with a 5000 years history since the bronze age, something new is still possible in metallurgy. An extension to this question is whether novelties in metallurgy is still necessary for our society. The questioning here is contrasted with materials that may appear to the public as the product of (more) modern science (and technology), for example, superconducting ceramic, liquid crystals, functional glass, blue LED, photovoltaic silicon, conductive polymers, carbon nanotubes, and graphene. One (metallurgists) may wonder why such a feeling. It is true that metallic alloys are common to everyone, present everywhere in our environment, in car, plane, building, devices. In addition, among all exceptional nonmetallic materials, some are recent discoveries highly

Y. Champion (*) Univ. Grenoble Alpes, CNRS, Grenoble INP, SIMaP, Grenoble, France e-mail: [email protected] © The Minerals, Metals & Materials Society 2022 H.-J. Fecht, M. Mohr (eds.), Metallurgy in Space, The Minerals, Metals & Materials Series, https://doi.org/10.1007/978-3-030-89784-0_21

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Fig. 21.1 The conflict between fracture elongation and yield stress. Different classes of main metal-based alloys show reduction in fracture elongation with increasing yield stress by the various hardening mechanisms

publicized for the disruptions they produce in their properties and potential applications or by prestigious awards. Note that there are only two Nobel Prizes strictly in the field of metallurgy, Charles Edouard Guillaume in 1920 for INVAR (FeNi alloy) and Dan Shechtman in 2011 for metallic quasicrystals. Without claiming being exhaustive, another reason is sometimes coming from a confusion between material and product. Glass is an excellent example, nearly as old as bronze, discovered by the Mesopotamians about BC 4500 and intensively developed by the Egyptian about BC 3000. Nowadays, window glass is an incredible material allowing tactile transmission of information to the smartphone, while an association with an invisible tinindium-oxide layer performs the function. Metallurgy has a long history and has contributed to mainstream advances in materials science and technology. It has produced for decades tremendous amounts of data, methods, experiences, and expertise, transferred to other phases studies and processing (ceramic, glass, semiconductors, etc.). One may emphasize the rather highly flexible character of metals and metallic alloys. Low- to medium-range melting points and plasticity allow large varieties of thermomechanical treatments and forming. Phase transformations are contributing to a large range of microstructures, phases, and properties. Possibilities in chemical combination lead to extremely large varieties of alloys from simple solid solutions to very complex ones; the large variety of steels is an emblematic example. The flexibility of metallic alloys is one of the origins of their strong impact on industry, economy, and society development and improvements, and ruptures in the field have still a lot to do. To illustrate, the main challenge in metallurgy is probably to overpass the conflict between strength

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and toughness [2] (Fig. 21.1), but there are many others, such as conflict between strength and electrical conductivity. HEA are part of new materials discovered in 2004 simultaneously in Taiwan and Oxford [3, 4]. They are also in some ways the products of a new concept in the formation of metallic alloys [5], which open new perspectives for novel properties and forthcoming applications. In this chapter, we shall address an overview on thermophysical properties of the HEA and CCA, in particular those measured in parabolic flight, and emphasis of the necessary need of these properties for future theoretical understanding and technical developments. The chapter is introduced by a short overview on metallic alloys design and novelty brought by the HEA and CCA as well as their main properties.

2 Metallic Alloys and Alloy Design The ranges of metallic materials are so large that alloys are traditionally separated in classes with their specificities and their science and engineering experts groups. To make it more complex, the classes are changing, depending on the scientific and technological domains. For materials science, the class is the alloy composition and microstructure, often in relation with the mechanical properties (with subclasses for chemical and physical metallurgy, mechanical properties): pure metal, solid solution, and multiphase alloy. For engineering, the class is related to alloys processing, depending on melting point range and forming ability. For the physics or functional applications, classes are physical properties: magnetic (with subclasses soft, hard, ferromagnetic alloys, etc.), electric, dielectric, etc. Nevertheless, it is fortunate that all agree on a classification based on the main metal constituting an alloy family: iron-based alloys (note that steel, Fe with small C content constitutes its own class), aluminum (with industrial subclasses: 1000 series, 2000, 8000, etc.), titanium, copper, and nickel. It is emphasized that such classification is a strong feature of the standard approach of the alloys design. The principle consists in adding elements to the base metal, starting from low content and gradually increasing it. Initially, this is done empirically through trial and error, and then through long-term experience. The method lasts since the bronze age with addition of tin to copper to make it stronger. It suggests quasi-infinite possibilities and is guided by desired properties. Hence, appropriate additions identified in the long history of metallurgy have led, after discovery and incremental improvement, to the all-common alloys used today. Sometimes, some alloys with exceptional properties were obtained by chance and lost, as for the case of the Damascus steel (wootz) [6]. But technically high standard alloys are the results of long-term research and development. Steel is probably one of the most extraordinary class of alloys. The large variety of its metallurgical states is based on the unusual bcc low-temperaturefcc high-temperature allotropic transition and large difference in carbon solubility between the two crystallographic states. Then stabilizing fcc austenite at lower temperature by adding nickel, varying thermal treatment to produce martensite and

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bainite, and adding chromium for oxidation resistance (stainless steel) generate diversity. With time, appropriate additions led to complex microstructures, duplex (austenite/ferrite), dual phase (ferrite/martensite), and TWIP/TRIP alloys. Browsing the various classes of modern metallic alloys, one notices their high degree of chemical and structural complexities. Duplex steels contain up to 10 elements with fractions from about 0.1 to 22 atomic %. The bcc βTi are quaternary and quinary alloys with vanadium content up to 15 atomic %. The new generation βTi for medical applications contains up to 29 atomic % of Nb, 13% Ta, and 4.6% Zr [7]. In the search for novelty, elemental addition has been extended to metallic compounds. The most emblematic example is the intermetallic NiAl superalloys, with high creep resistance for turbine blades of jet engines or steam turbines. Nowadays, active researches are devoted to TiAl and other alloys are of interest for their high strength such as FeAl. Motivation as starting point for alloys development is also based on specific property of compounds. To illustrate, a first example is the metallic glass [8, 9]. These are multi-elementary metallic materials obtained by rapid cooling (from 10 to 106 K
s1) to avoid crystallization and then forming long-range atomic disordered structures. These alloys are brittle but present extreme strength. Most metallic glasses are generated from a so-called deep eutectic in a binary phase diagram, that is, that the eutectic temperature is low compared to melting points of the constituting elements. High viscosity in the supercooled domain and eutectic structure alternating the two elements are much favorable to form disordering. Then strategy of elements additions is applied to the eutectic composition as for addition to a base metal, to improve the properties. The CuZr eutectic has been intensively studied producing amorphous which glass-forming ability is gradually improved by Al addition up to 10 atomic %. Then subsequent addition of Ti and Ni led to ZrCu-based bulk metallic glass giving part with few centimeters dimensions. Other metallic glasses were discovered not based on eutectic but still characterized by high viscosity in the supercooled domain related to metal-metalloid interactions. The starting point for metallurgical development of this class of alloy is then the property of “glass-forming ability.” A second example is the shape memory alloy that shows the property of shape recovery after plastic deformation by thermal treatment. Here, the starting point is an alloy composition exhibiting the martensitic transformation. The reference alloy finding dental applications is NiTi. Then various alloying has been studied to focus on the desired properties leading, for example, to high-concentration NiTiPd, NiTiPt, NiTiHf, etc. [10]. To sum up, alloy design strategy can be defined as gradual additions of atomic elements to a main metal or a compound, generally a binary alloy showing specific properties. The main metal/simple compounds constitute the solvent. It is emphasized that the processing in the design process is based on the main metal (or the simple compounds) properties (thermophysical, mechanical, physical, and chemical). As one has seen, metallic alloys are already very complex in terms of composition and microstructure (Fig. 21.2). So what is different with high-entropy alloy and derived complex concentrated alloys?

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Fig. 21.2 Metallic alloys are already in complexity. Illustration with variation in phases and composition with thermal treatments in martensitic stainless steel 15-5-PH observed with atom probe tomography. (Adapted from Couturier Materialia 2020) Fig. 21.3 Illustration of alloy design strategies on a ternary phase diagram for simplification. Traditional alloys start from a corner or specific composition of the phase diagram. HEA and CCA start from a solid solution in the middle of the phase diagram

For HEA and CCA alloys, the design is starting from equi-atomic multielementary solid solution (quinary alloys and more) which constitutes the solvent. Then the properties are probed by variations of the HEA composition and elementary additions from the “middle” of the phase diagram while “traditional” alloys are probed from the corner or specific composition. The two different approaches are schemed in Fig. 21.3. This is a clear rupture in metallic alloy design and a novel fertile ground for innovation since the solvent has new complex properties in relation to multi-atomic interactions. The emblematic example of HEA and the first one discovered by Cantor and collaborators is FeCrMnCoNi [4]. The same year, Yeh and collaborators studied series of multi-elementary solid solutions and proposed the

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name of high-entropy alloy for this class of alloys [3]. In a critical review on HEA and CCA, Gorsse, Couzinié, and Miracle pointed out the difference in motivation of the two groups [5]. The group of Oxford focused on the unexplored central region of the phase diagram and the fact that multicomponent, highly concentrated alloys open to extreme large numbers of new alloys. The group of Taiwan was working on possibilities from thermodynamics considerations to avoid intermetallics formation, responsible of low fracture resistance in alloys solid solution. The two motivations eventually converged to a unique new alloy concept. In some sense and unusually, a new material concept came from two different origins as pointed out by Miracle [11]. Design of HEA and CCA needs to deal to their chemical complexity to which must be added the timescale in materials discovery. The world and its societal issues need “smart” and ethical (friendly, recyclable, etc.) materials in shorter and shorter timescale. Hence, alloy design must be thought in an “accelerated form” which needs development and implementation of up-to-date combinatorial high throughput experimental techniques [12], supported by artificial intelligence tools and modelling such as DFT [13], thermophysical data calculation, phase diagram CALPHAD modelling [14, 15], etc.

3 Definitions 3.1

High-Entropy Alloys

The idea of the Oxford group was exploration of the very large possibilities in composition and phases of multi-elementary and concentrated regions of the phase diagrams. From that, a first definition is that alloys are composed of five or more elements with atom fraction between 0.05% and 35%. In the other approach of the Taiwan group, HEA are derived from a stricter criterion, that is, that the formation of solid solution is predominant over intermetallic compounds. Here is one of the main motivations in metallurgy, which is avoiding intermetallic formation well known for alloy embrittlement effect. Some quantification is then derivable from the free enthalpy of phase formation in thermodynamic equilibrium condition, ΔG ¼ ΔH  TΔS. In an ideal view, as assumed by Yeh and collaborators [3], solid solution formation is dominated by the entropy, ΔGss   TΔSss. In contrast for ordered intermetallic ΔSim  0 and the formation is dominated by the mixing enthalpy ΔGim  ΔHim. It naturally results that solid solution is as much favored as TΔSss is much more negative than ΔHim. Ideal solid solution means that the entropy is given by the ideal configurational contribution uniquely defined by the Boltzmann relation: Sc ¼ k ln Ω, with k the Boltzmann constant and Ω the number of microstates configurations. For an alloy composed of n elements with atom fraction xi of the ith, the ideal n P configurational entropy is Sc ¼ k xi ln xi . One notices that the maximum of Sc is 1

given by 8i and j, xi ¼ xi and Sc ¼ k ln n. Then Sc is increasing with the number of elements following, respectively, n ¼ 4, 5, and 6 and Sc ¼ 1.39k, 1.61k, and 1.79k. From

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such consideration, Yeh and collaborators have proposed the condition that HEA is solid solution that respects Sc>1.5k. Strictly speaking, equi-atomic HEAs need at least five elements to respect the definition. However, increasing the number of elements allows respecting the rule with some deviation from equi-atomic composition. This joins the Oxford definition and is the first step toward CCA. Again, as already emphasized for these alloys design, definition shows the dominant role of thermodynamics in HEA.

3.2

The Four Core Effects

The multi-elementary and highly concentrated composition leads to four specificities for the HEA, called the four core effects. Originally conjectured from the HEA atomic structure, these effects were the subject of numerous discussions and controversies since HEA discovery. The extended number of works so far have seem to comfort some of the original ideas, bringing interesting perspectives as discussed in detail by Miracle [11].

3.2.1

The High Entropy

A first core effect is straightforward since it is deduced from the definition, which is that HEA is as stable as its configurational entropy is high. It is here an opportunity to comment this strict definition with respect to possibility in HEA solid solution formation. Again, in such ideal view, HEA pioneers acknowledge that high entropy would not be able to compete with extreme stable compounds having large negative formation enthalpies (in the range of 30 kJ/mol to 120 kJ/mol) such as carbides (SiC, TaC, TiC), boride (TaB2), nitrides (Si3N4), etc. Another strong fact discussed by Miracle [11] is the decreasing probability of forming solid solution or increasing probability of binary intermetallics formation with increase of the number of elements, which is contrary to the high-entropy core effect. This is supported by experimental observations and modellings and can be simply argued by that facts that with increasing the number of elements, the configurational entropy is increasing more slowly following ln n than the number of potential binary systems, varying following n/2(n  1). An interesting experiment has consisted in substituting successively one element of the Cantor FeCrMnCoNi and then keeping the configurational entropy [16]. In this work, substitutions (Ti for Co, Mo or V for Cr, V for Fe, and Cu for Ni) were such that elements have the same room temperature, structure, size, and electronegativity. Multiphase formation observed proved that entropy is not alone the criterion for solid solution formation and that enthalpy has to be considered. From the pioneering work of Yeh and Cantor, other criteria have then added to design HEA derived from the empirical Hume-Rothery rules for binary solid solutions formation and based on the following defined parameters:

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– The mixing enthalpy: ΔH mix ¼

n P P n i¼1

ij j¼iþ1 4ci c j ΔH mix

n is the number of elements constituting the alloy. ci and cj are the i and j elements content. ΔH ijmix is the mixing enthalpy of the binary alloy i and j. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Pn  ri 2 c 1  – The atomic size difference: δ ¼ 100 i i¼1 r r i and r, respectively, are the atomic radius of element i and average atomic radius. P – The valence electron concentration: VEC ¼ ni¼1 ci VECi VECi is the valence electron concentration of element i. These parameters would predict favorable solid solutions formation [17] but in a very empirical way still although they constitute a first evaluation and guide to start in the so-complex domain.

3.2.2

The Lattice Distortion

Lattice distortions are responsible for properties variations in pure metals, and the phenomenon is exploited for adjusting and improving properties since the origin of metallic alloys development. It is also sometimes responsible for unwanted degrading of properties. With respect to the pure metal with perfect lattice as depicted in Fig. 21.4a, distortion is produced by the difference in size between the solvent atoms and the added solute one. As shown in Fig. 21.4b, with the example of a substitution solid solution, the orange atom is larger than the blue hosts producing long-range displacement. Lattice distortion is well known for strengthening; it is shown that the yield stress varies with solute atom concentration, c following, Ys / Ecn, with E the solvent Young modulus and the exponent 1/3 < n < 1 depending on atomic interactions and concentration range. The necessary increase of stress for plasticity to occur is explained by larger difficulty for dislocations to move in the long-range distorted lattice. To pass, the distortion dislocation must bend increasing its length, the line tension, and then energy.

Fig. 21.4 Schematic illustration of the lattice distortion hypothesis. (a) Pure metal with perfect lattice. (b) Diluted substitutional solid solution with long-range distortion gradient produced by a larger atom in size. (c) Short-range (atomic distance) and homogenous distortions in HEA

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Fig. 21.5 Strengthening of Au-Ag solid solution

In the model system where Au and Ag are miscible in all content, strength varies linearly when Ag (respectively Au) is solvent and Au (respectively Ag) is solute, reaching the maximum strength for 50 atomic % (Fig. 21.5). In HEA, the lattice distortion is due to atomic size difference at each node of the crystal lattice as schemed in Fig. 21.4c. Crucial difference with “standard” dilute solid solution is that there is no quantifiable distortion gradient. Distortion is true at any point, corresponding to small difference from perfect lattice translation. It is interesting to note that as distortion means difference from a reference perfect lattice, HEA distortion should be then redefined at any point of the crystal as the difference between two adjacent cells. But a full description would then need to define variations in this cells difference at each point of the crystal lattice. In a more simple way, this is what happened for Au-Ag when the alloy is close to 50 at %. On any side of the plot (Fig. 21.5), none of the metals is the relevant reference. Easier way of “quantification” is an evaluation of lattice distortion with respect to an average crystal lattice, which can be determined from the maximum of X-ray diffraction peaks, for example. The average value of lattice distortion is then given by the relation previously mentioned forffi designing alloys with respect to Humeqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  Pn  ri 2 Rothery rules, δ ¼ 100 i¼1 ci 1  r . Quantitative structural characterization is obviously required for properties understanding, and specific analyses might be useful as the pair distribution function [18]. Another disruptive approach would be to describe the alloy from the local energy variations (potential energy landscape, PEL) ignoring atoms as proposed, for example, for glass description [19, 20]. It is emphasized here that though HEA are crystals, their structural definition contains a statistical component, which naturally

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affects their properties. This statistical component is a supplementary parameter, compared to “standard” alloys, which could advantageously be used for novel properties development as far as it is understood and controlled.

3.2.3

Sluggish Diffusion

Diffusion is a thermally activated process occurring owing to the formation of an atomic and vacancies gradient and displacement by atomic jump into a nearby vacancy. Atomic diffusion  isQ characterized by the temperature-dependent diffusion coefficient: D ¼ D0 exp  RT , D0 is the jump frequency, Q is the activation energy, and R is the gas constant. Roughly, D0 and Q are as low as disorder is large in crystal lattice. One feels that jump is more difficult from place to place, when the atomic position (lattice translation) is not well defined. In addition, activation energy related to bonding energy should be lower in disorder lattice where there are less bonds to cut for vacancy formation. A good illustration is the difference between diffusion coefficient in bulk crystal and  grain  boundary. Accordingly, with possibly decrease Q of D0 and increase of exp  RT , predictions are not straightforward. To this, it is emphasized that disorder in HEA is not only of structural type but also of short-range atomic change and then of energy density disorder type (nonperiodic potential energy landscape). Sluggish diffusion was originally attributed to lattice distortions and observations of nanocrystals in as cast HEA, elevated recrystallization temperature, and formation of amorphous phase [21]. The first relevant measurements on the FeCoCrMnNi HEA were reported in 2013 [22] and, though seemingly proving sluggish diffusion, was the subject of controversy. The couple method was used to measure diffusion coefficient of each metal in the HEA and comparison with diffusion in the reference metal. Systematic higher activation energy was found as well as higher normalized activation energy Q/Tm where Tm is the melting temperature. This work showed that Q/Tm is increasing with the number of elements in the alloy in connection with higher fluctuation of the lattice potential energy. Measuring diffusion in complex alloys and comparison with appropriate references are very challenging. Various investigations and analyses, so far, seem to show that in absolute, diffusion in HEA can be lower or higher and a sluggish effect is not only related to the number of elements. However, as generally observed, the mixings show lower Tm and higher resulting Q/Tm which, in this normalized context, places HEA sluggish for diffusion (see the comment in [11]).

3.2.4

The Cocktail Effect

This is probably the most intriguing effect and the one that cannot be quantifiable. Cocktail effect means the birth of unpredictable properties from the mixing of elements. In other words, the properties of the mixing are not predictable from the

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properties of each component. Such effect occurring from complex mixing was first emphasized by Ranganathan [23]. In this article, the cocktail effect is not directly mentioned, but the author gives an interesting opinion at the time, on relatively novel metallurgy approach based on complex multi-elementary mixing, he called multimetallic cocktail, with the examples of metallic glasses, gum metals, and newly discovered HEA. An interesting and straightforward illustration was reported by Laurent-Brocq and collaborators [24]. Lattice parameter variation was studied from deviation of the “Cantor” alloy NiFeCrCoMn on the alloy series Ni1-4x(FeCrCoMn)x. From the binary alloys NiA, with A ¼ Fe, Cr, Co, Mn and the mixture of the later four metals, the linear Vegard’s law should be expected. As shown in Fig. 21.6, except for Co, the other metals lie close onto the Vegard trend. However, experimental measurements using X-ray diffraction show deviation as soon as 40 atomic % of (FeCrCoMn) seemingly indicating a strong influence of Co. This might be pure feeling, and the lattice parameter variation is most likely depending on the cocktail mixture properties. It must be emphasized that in their study, authors verified absence of magnetic property effect as well as consideration of bcc structure of Cr. The work also shows a decrease of the Young modulus with increasing the four (FeCrCoMn) elements content. Another peculiar behavior, considering that for the pure element having close Young moduli, a decrease of lattice cell should predict an increase of the alloy Young modulus (increase of the local energy density).

3.3

Toward Complex Concentrated Alloys

Soon after the discovery of HEAs, a solid solution of five or more equi-atomic elements was found to be very restrictive and potentially limiting in discovery of interesting high prospect alloys. Research focuses on the improvement and Fig. 21.6 Deviation from Vegard’s law for Ni14x(FeCrCoMn)x alloys. (From Ref. [24])

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discovery of new properties. It has been found more productive releasing certain restrictions in particular on the composition and phase formation while retaining the HEA approach of studying the alloy from the middle of the phase diagram (no principal element), as shown in Fig. 21.3. For example, study of deviation in Ni content from the exact “Cantor” NiFeCrCoMn alloy shows that the maximum hardness is obtained for 60 atomic % of Ni instead of the maximum entropy at 20% (Fig. 21.7) [24]. HEA then derived toward complex concentrated alloys (CCA) also called sometimes multi-principal element alloys (MPEA). Coupling the concentration concept to element addition, phase mixing properties (as in dual phase), and phase transformation ext. leads to extremely wide domain of alloys just waiting for being investigated. A recent article reports on a multi-concept approach conjugating face-centered cubic (fcc) and body-centered cubic (bcc) phases, grain refinement, and transformationinduced plasticity (TRIP) effect in a Ti35Zr27.5Hf27.5Nb5Ta5 CCA (Fig. 21.8). This CCA shows high strength in the ranges of 950–1150 MPa and reasonable ductility of 12%.

4 HEA and CCA Properties and Perspectives There is obviously no pretention to detail all properties of the HEA and CCA in this paragraph, and the readers will find appropriate reviews on the various topics. The objective here is to show perspectives focusing mainly on mechanical (structural) and functional properties. It must be emphasized again that HEA and CCA are still novel metallic alloys, and then the results are still scarce not showing obvious trends Fig. 21.7 Hardness as function of the composition for Ni1-4x(FeCrCoMn)x alloys. (From Ref. [24]) Hardness H (GPa)

4

3 Mott-Nabarro-Labush H = 1.91 + 0.73 · (4x)2/3 R 2 = 0.92 2 Nanoindentation MNL 1 0

20

40

60

80

4x = [Cr] + [Mn] + [Fe] + [Co](at. %)

100

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Fig. 21.8 Ti35Zr27.5Hf27.5Nb5Ta5 CCA formed of nano-grains of fcc phase in red and bcc phase in blue with various size depending on the annealing time at 650  C. Scanning electron microscopy in electron backscatter diffraction mode. (From Ref. [25])

and straightforward comparison with standard alloys. It is also currently a natural trend to (too soon to my opinion) envisage applications for any kind of materials, and then questioning on sustainability and recycling are quickly arising. HEA and CCA are in their teen age, and deep evaluation regarding these aspects is most likely premature. However, it is clear that in the future, properties such as resistance to oxidation, corrosion, fatigue, etc., will be dominant issues. This book is devoted to measurements carried out in microgravity on metallic alloys. I then made the choice to focus in this paragraph on HEA and CCA for hydrogen storage, since alloys for such application are studied in microgravity. Eventually and also in relation with the book topic, the thermophysical properties of HEA and CCA will be the subject of a specific paragraph ending this chapter.

4.1

Mechanical Properties

Mechanical properties are dominant in metallurgy since alloys find major use as structural materials as well as in applications where mechanical integrity (fatigue, plasticity, etc.) is required (electrotechnics, microelectronics, packaging, etc.). In their seminal paper [3], Yeh and collaborators report on hardness and yield strength of their newly discovered HEA showing this main concern. Solid solutions were historically produced to strengthen metals, and then as some sort of ultimate and complex solid solution, HEA have been soon seen promising for strength improvement. Then varying either composition or/and producing multiphase toward CCA should lead to even more mechanical behaviors improvement such as high strength, ductility, etc. As a new alloy concept, HEA and CCA have been soon regarded as potential candidates to meet issues in relation to energy saving in transportation, for example. Higher temperature and more efficient airplane engines need novel alloys

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Fig. 21.9 Scheme of two successive cross sections, (a) and (b), perpendicular to a dislocation line showing that environment around the line is continuously changing. This is producing fluctuation of the Burgers vector and the shear modulus. (c) is a scheme of a dislocation line with the fluctuating Burgers vector

to replace superalloys, and lighter and high-strength alloys are necessary for structure lightening. As previously discussed, atomic structure of HEA/CCA is characterized by lattice distortion. In addition, although atomic organization is not clearly defined (possible local clusters formation, nano-phases, etc.), it should be close to a random atom distribution on the crystal lattice (maximum configurational entropy). From that description, all defects involved in the micro-mechanisms controlling the mechanical properties are to be revisited with consideration of the small and local differences with respect to an average crystal lattice. High-temperature diffusion-like mechanism has been discussed in the “sluggish diffusion section.” At low temperature, the deformation is controlled by dislocation dynamics. There are two interesting structural aspects concerning this defect. First, if a dislocation line is well defined on an average crystal lattice, the distortion along the line is producing fluctuation in the magnitude of an average Burgers vector such that the Burgers vector b varies continuously along the line by a small amount comprised in δb (Fig. 21.9). This means that dislocation energy, which is also modulated by a fluctuating shear modulus, varies along the dislocation line, E  (G  δG)
(b  δb)2. A second feature is that there is no conservation of the local chemical structure of the moving dislocation at any point of the line and then no local energy conservation. This can be seen as a solute-(average) dislocation interaction (as punctually in a standard alloy) occurring continuously. Varvenne and collaborators proposed such description to model the yield strength with good comparison to the fcc NiFeCrCoMn HEA properties [26]. These authors have also examined interaction of dislocation with dilute solute atom in the HEA medium [27]. Here is pointed out the principal feature of plasticity and strengthening that is dislocation interactions with structural characteristics most likely all affected by distortions and atomic random distribution. The work hardening (Taylor relation) is controlled by

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Fig. 21.10 Scheme of (a) grain boundary and (b) twin boundary

dislocation-dislocation interaction. The Hall-Petch (grain size) effect is controlled by dislocation-grain boundary interaction. The deformation twinning is controlled by dislocation dissociation and stacking fault energy. The grain and twin boundaries are characterized by a lattice disorder but not chemical disorder with respect to the reference random atom distribution (Fig. 21.10). One notices that the stacking fault energy reported for the HEA NiFeCrCoMn is consistent with the observation of decreasing the value with alloying. Stacking fault energy for this alloy is measured of about 20–30 mJ/m2, for comparison Ni is 90 mJ/m2. From all these structural features, different behaviors and/or better performance with respect to common alloys have to be expected. As mentioned in main reviews [1, 28], direct comparison of available data from various HEA and CCA is still difficult owing to difference in composition and thermomechanical treatments. The most recent review on the mechanical properties reports on an extended data analysis available since 2019 [28], and one focuses only from this review on the emblematic strength-deformation behavior. Fig. 21.11 from [28] shows that HEA and CCA are mapping all the domain of the tensile strength vs. elongation properties of common metallic alloys. In compression, the properties are far better with larger values for compression strength up to twice as large (4 GPa), with still reasonable deformation (30%). These results may be seen modest without the expected clear rupture, but it must be emphasized again that the field of HEA/CCA is still young and that time is needed to find relevant exploitation of all the novel structural features. In support of this argument, probably one of the most remarkable results has been reported by Gludovatz et al. [29]. It is well known that by any process, including by lowering the temperature, the increase in resistance leads to a decrease in ductility of alloys, due to the reduction in dislocations activity, decreasing work hardening and then earlier localization. These authors carried out tensile tests on the NiFeCrCoMn HEA at 293 K, 200 K, and 77 K and showed unexpected (cocktail effect) improved properties with increase in strength and elongation with decreasing temperature

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Fig. 21.11 Tensile/compressive strength vs. elongation/compressive strain for various HEA and CCA compared to standard alloys. (From Ref. [28])

Fig. 21.12 Tensile stress vs. strain for HEA at 293 K, 200 K, and 77 K. (From Ref. [29])

(Fig. 21.12). Constant fracture toughness in the range of 220 MPa
m1/2 is also reported. These properties are associated to a change in deformation mechanism from dislocation glide at room temperature to gradually nano-twinning at 77 K which the atomic structure of HEA is able to do. Obviously, other such unexpected behaviors are to be found.

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Functional Properties

In their critical review [1], Miracle and Senkov pointed out that main functional properties are reported on the 3D transition metal alloys family. Hence, one finds some results on electrical (conductivity), thermal (conductivity), magnetic (soft), and chemical for catalysts and reaction (mainly hydrogen storage). This latter is detailed in the next specific section. With combined effects, one finds thermoelectricity, shape memory, irradiation resistance, functional films for diffusion barrier, and solar absorber. As for mechanical properties, specificities and difference with standard metallic alloys are attributed to atomic distribution and lattice distortions. Main references for detailed reviews on functional properties are reported in [1] with an interesting viewpoint in [30]. Electrical resistivity investigated on AlxCoCrFeNi (0  x  2) is the range of 100–200 μΩ
cm. This is similar to the resistivity of the Vitreloy, Zr-based metallic glass, and two order of magnitude larger than the reference copper 1.7 μΩ
cm. For other comparison, bronze (CuSn) alloy has resistivity of 42 μΩ
cm. The electrical resistivity varies linearly with the temperature. Linear variation is also observed with Al content but with a gradual change owing to crystallographic transition from FCC to BCC. Other element additions have been studied such as Ti in AlxCoCrFeNi. These alloys show large variation in the resistivity from 60 to 400 μΩ
cm depending on annealing. Interpretation in terms of HEA and/or CCA properties is not straightforward owing to the formation of B2 and various intermetallic phases. Refractory alloys have been studied such as Hf8Nb33Ta34Ti11Zr14, showing resistivity of 46 μΩ. cm at room temperature and 36 μΩ
cm at 8K and then drop to zero at 7.3 K. Thermal conductivity was studied in AlxCoCrFeNi (0  x  2) annealed at 1273K and water quenched and as cast AlxCrMnFe1.5Ni0.5Moy (x ¼ 0.3, 0.5 and y ¼ 0, 0.1). Respectively, thermal conductivity and diffusivity of these alloys are in the range of 10–27 Wm1k1 and 2.8–3.5 mm2s1. These values are lower than pure metals but similar to steels and Ni superalloys. The thermal conductivity decreases with Al content, which is attributed to increase in the lattice distortions. Linear thermal expansion is rather classical compared to highly alloyed metallic materials such as austenitic steels. Soft magnetic properties are investigated for electrical transformer application, with search for the combination of low coercive field, high-saturation induction, and high electrical conductivity for low loss at high frequency. Then the starting point is naturally FCC solid solutions of Fe, Ni, and Co alloys. The complexity toward CCA has been found by adding Al to form FCC + BCC/B2 phase and adding Si to form FCC + silicides. These alloys are all ferromagnetic with, respectively, saturation magnetization (Ms) of 102 emu/g and 80.5 emu/g with Al and Si additions. For comparison, Ms is 151 emu/g for FeNiCo, 218 emu/g for Fe, and 55 emu/g for Ni. The complex FeNiCoAl and FeNiCoSi alloys show very small magnetostriction effect which is of particular interest when subjected to external magnetic field. More complexity was tested with combining FeNiCo with AlCrCu leading to AlCoCrCuFeNi ferromagnetic in the as-cast state. The alloy exhibits very complex

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distribution of paramagnetic FCC Cu and AlNi B2 phases and ferromagnetic FCC CoCrFe and BCC CrFe phases. The mixing of AlxCrTi to FeNiCo (0  x  2) produces as-cast ferromagnetic with also complex phase mixing.

4.3

Hydrogen Storage

Hydrogen is the most efficient and potentially a clean energy carrier for transportation, nomad, and static devices. Currently (twenty twenties), researches in relation with hydrogen know unprecedented funding around the world for technological development related to energy and ecological societal needs. The growing number of hydrogen fuel cell public buses, trains, and private cars in European countries is emblematic. For improving and increasing hydrogen use, hydrogen storage is probably among the key challenges with at least the following specifications: the carrier must be efficient for storage and restitution, sustainable, high capacity, safe, and lightweight for mobile applications. Several technologies involving materials are studied (see, e.g., for review [31, 32]) as porous materials (metal-organic frameworks), liquid hydrides (cyclic alkane compounds, ammonia), and metallic hydrides such as aluminum-based and Mg-based alloys and intermetallics (ZrNi). Discovery of LaNi5 led to significant development of solid materials for this application. The intermetallic storage is reversible, occurring around atmospheric pressure and with reasonable capacity of 1 H/M (hydrogen per metal). The conditions allow using LaNi5 as negative electrode in batteries, electrochemically charged in hydrogen. However, in spite of excellent capacity, the performance is decreasing drastically with cycling, owing to decrepitating-related stress during hydrogenation and release and then corrosion in the KOH electrolyte. In the search for improving sustainability, a first level of complexity was used with partial substitution of La and Ni leading to La0.8Nd0.2Ni2.5Co2.4Si0.1 showing drastic improvement in cycling with less decrepitating. In this context, HEA and mainly CCA have appeared as potential interesting solutions for solid hydrogen storage materials because of their structural and chemical flexibility and tunable properties. In particular, the local lattice strain and fluctuation should possibly allow large interstitials sites for hydrogen occupation. The lattice strain may also allow higher resistance to decrepitating with less cyclinginduced strain. In addition, metallic elements mixing in solid solution is more easily adjusted for hydrogen affinity compared to intermetallics. In connection with composition and then a relevant parameter for designing, the valence electron concentration (VEC) shows correlation with hydrogen storage capacity. Generally, degrading capacity is known for increasing the VEC above a certain limit for bcc alloys [33]. From experimental works on various ternary, quaternary, and quinary refractory alloys, a linear correlation was observed between the VEC and hydrides stability [34]. Among a simple capacity indicator, VEC and its relation with electronic structure opens interesting starting point for understanding hydrogen sorption properties in HEA and CCA. In contrast, the relationship between the lattice

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distortion and capacity is not so clear and would need further investigations. For example, the group of Nigard studied series of bcc alloys TiVZr(1x)NbTax, where the lattice distortion is tuned by the Zr content. Nevertheless, interpretation is not straightforward since the various alloys have different lattice parameter and stability, with some alloys showing phase separation after hydrogen sorption/desorption [35]. Perspectives are now obvious, though research works are still few and focused mainly on lightweight bcc HEA. The first work was reported in 2014 by Kunce et al. on the bcc solid solution TiZrNbMoV showing low capacity of 0.6 weight % [36]. Soon after, Sahlberg et al. reported on an outstanding capacity of 2.5 H/M (2.7 weight %), larger than capacity of the single elements (2 H/M) for the TiVZrNbHf alloy [37]. Interestingly, hydrogenation is a single step reversible bcc-bct reaction, and such high hydrogen uptake has never been observed in transition metal, indicating that tetrahedral and octahedral sites are occupied in the crystal lattice [38]. Initially, researches focused on elemental substitution in HEA revealing interesting phenomena. For example, isoelectronic change of V by Ta leads to TiTaZrNbHf showing unexpectedly two-phase transformations bcc-bct-fcc in hydrides formation [39]. Such type of results incite to extend toward the CCAs for improving, with elements content variations such as Zr-deficient TiVNb-Zr [40] and elemental doping with the same VEC as TiVNb-(Cr, Ni, or Co) [41]. Interestingly, comparisons between the various alloys in terms of hydrogen capacity and structural properties (phase transformations) put the lattice distortion at the first level in dominant parameters along with VEC for designing relevant hydrogen storage materials. Beside alloy design, preparation, processing, implementation, and integration will be soon a great challenge for the development of HEA/CCA. Alloys containing light metals like in MgZrTiFe0.5Co0.5Ni0.5 [42] and MgAlTiFeNi [43] are rather challenging. The difference in melting points between elements and high-vapor tension of Mg is dealt in these works by solid-state ball milling syntheses. The refractory alloys containing Ti, V, Zr, Nb, Hf, Mo, and Ta are prepared by melting and solidification, for which thermophysical properties (viscosity, thermal capacity, surface tension, etc.) are key data. As a general remark, it must be emphasized that the capacity of the refractory HEA/CCA is similar to those of rare earth metal hydrides (H/M >2.3), which are strategic elements and have a high geopolitical issue.

5 Thermophysical Properties Previous pages attempt to define HEA and CCA and to show their novelties and perspectives. As any metallic alloys, their development with specific performances will need to control processing steps. The properties strongly depend on the history of the material, and solidification is one of the crucial steps for metallic alloys all the more crucial with the chemical complexity of the alloys. During solidification,

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structural evolution and conditions will lead to final microstructure and phase formation with various possible features such as grain size, micro- and macrosegregation, dendrite, clustering, gradient, phase separation, stress relaxation, growth twinning, etc. Understanding structural evolution during solidification demands the knowledge of thermophysical properties of the melt. Additionally, the necessary precise thermophysical properties for theory and modelling require measuring them in microgravity [44]. This justifies the paramount importance of parabolic flight and space experiments. Thermophysical properties of HEA and CCA are still very few. Ground experiments are reported for temperature surface tension. For example, on CuSnBiInPb [45], the same group in Russia published viscosity measurement on the same alloy in Russian language only [46]. The available data for HEA/CCA from microgravity experiments are reported in the next paragraph. With their specific multi-elementary high concentration, character-specific behaviors are expected. With available techniques (described in this book), measurements are for surface tension as function of temperature and undercooling, specific heat, thermal conductivity, emissivity, heat capacity, electrical conductivity, density, coefficient of thermal expansion, and viscosity.

5.1

Parabolic Flight Experiments

The very first experiments on HEA were carried out during the DLR/ESA (German space center/European space agency) parabolic flight campaign 2018. The studies were supervised by M. Mohr, R. K. Wunderlich, and H. J. Fecht of the Institute of Functional Nanosystems of Ulm University and part of the DLR ThermoLab and ESA ThermoProp projects. The works were performed on a FeCrNiCo-0.1Al alloy with three parabolas to measure the surface tension and viscosity undercooling of about 100 K and one parabola for electrical resistivity and sample radius measurements. The equipment is consisting in a process and a sample chamber, connected to a vacuum pump system and a gas circulation unit. The sample is heated and positioned by radio frequency (rf-) electromagnetic fields. A dipole field for heating and a quadrupole field for positioning are superimposed using a single coil system operating at frequencies of 330 kHz (heating) and 170 kHz (positioning). The process chamber is equipped with two high-speed cameras. The axial camera is additionally equipped with an optical pyrometer with a measurement range between 300 and 2100  C. A second camera is positioned in radial direction. In addition, the sample coupling electronics (SCE), developed by Georg Lohöfer from DLR, Institut für Materialphysik im Weltraum, Cologne, is used to measure the current, voltage, phase shift, and frequency of the rf-heating voltage with high accuracy and a data rate of 400 Hz. Then the changed sample coupling due to the varying sample diameter during oscillations is measured to determine surface tension and viscosity.

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Fig. 21.13 Airplane for experiments close to Bordeaux, France, and representative temperaturetime profile during parabola flight

Parabolic flights allow about 20 s of reduced gravity, which is sufficient for most alloys to heat the sample until melting, overheat the melt, and cool it down until solidification. The cooling period is the time span where the heater is turned off, and therefore, only minimal forces act on the sample. Hence, short heater pulses are applied to initiate surface oscillations. The oscillation frequency is measured in order to obtain the surface tension, and the damping time constant of the surface oscillations is used to obtain the viscosity of the sample. The acceleration on the sample is shown as a function of time, in green in Fig. 21.13. The μ-g phase is about 20 s. The heater control voltage is shown in blue, showing the time duration when the sample is heated until melting, followed by further overheating in the beginning of the μ-g phase. Afterward, the heater is turned off and only two short pulses are used to initiate surface oscillations. In red, the sample temperature is shown as a function of time. After 10 s, the sample is already molten and overheated to the maximum temperature of around 1700  C. Afterward, the sample undercools by about 100 K and solidifies during levitation. From the three working parabolas, the first one was performed without heater pulses to be able to measure the sample electrical resistivity and diameter by the sample coupling electronics (SCE). The analysis of the electrical resistivity and the sample radius was done by Georg Lohöfer from DLR, Institut für Materialphysik im Weltraum, Cologne. The surface tension σ is evaluated from the surface oscillation frequency νR and the sample mass M using σ ¼ 38 πν2R M. However, applicability of this relation is subjected to some constraints. The surface oscillation frequency, characterized by νR the Rayleigh frequency, pertains to the oscillations of a force-free spherical droplet. Under 1-g electromagnetic levitation, the sample is not force free and deformed. A correction was developed to account for the effect of the magnetic pressure on the surface oscillation frequency and was successfully proven by comparison with micro-g ground-based surface

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Fig. 21.14 Results of the parabolic flight experiments as a function of temperature. (a) Surface tension (measured by the SCE and optical method), (b) viscosity, (c) resistivity and sample radius, and (d) mass density

tension measurements in an EML device. Under reduced gravity conditions such as present in a parabolic flight, the sample is nearly force free and spherical. Under this condition, application of the so-called Cummings and Blackburn correction results in a reduction of the surface tension values in the range between 2% and 3% when the measured surface oscillation frequency instead of νR is used in the formula for the evaluation of the surface tension. This correction was not yet applied to the data presented here. It is only weakly temperature dependent. The temperature dependance of the surface tension from Fig. 21.14a is given, with T in K by σ ðT Þ ¼ ð1:7559  0:004Þ  ð5:31  0:21Þ  104 ðT  1719Þ N
m1 The viscosity is determined by the damping time constant of the surface oscilla3 M 2 1 tions following η ¼ 20π , where R is the sample radius and is τ the time R νR τ constant. Small amount of data was obtained for viscosity of the HEA, due to the small numbers of parabolas with heater pulses (two). The temperature-dependent viscosity is shown in Fig. 21.14b. At liquidus temperature, the viscosity is

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η ¼ 33 mPa
s which is particularly high compared to viscosity of constituting elements at liquidus, 5.6 for Fe, 5.7 for Cr, 4.7 for Ni, and 5.4 for Co. Eventually, the electrical resistivity and sample radius as a function of temperature were measured, shown in Fig. 21.12c. It can be seen that both resistivity and expansion coefficient vary strongly between the solid and liquid phase. The measurement was done from the highest temperature during cooling into the undercooled liquid and in the solid phase. Specific resistivity at liquidus temperature is obtained of the order of ρ  164 μΩ
cm. From mass density derived from the sample radius variations, the mass density changed is obtained, ∂ρ/∂T   8.9  104 g
cm3. The first results and unusual thermophysical properties have convinced to pursuing in further investigations. In summer 2021, the FeCrNiCo quaternary alloy will be analyzed in the upcoming Batch#3 on board the ISS. Furthermore, at the same period, measurements will start on TiVCrZrNb HEA and Ti0.3V0.25Zr0.1Nb0.25Ta0.1 CCA for H-storage, in the 2021 parabolic flight campaign.

6 Concluding Remarks High-entropy alloys (HEA) and complex concentrated alloys (CCA) form a new class of metallic materials based on their design approach. Conventional in particular modern alloys can be complex in composition, microstructure, and phase distribution, but always, design is starting from a single metallic element (“corner” of the phase diagram). HEA and CCA bring a real breakthrough in design with, as a starting point, the multi-elementary mixing of at least five elements (middle of the phase diagram). This novel approach leads to tremendous possibilities, from which novel properties are expected in particular in relation with the “cocktail” core effect. In this chapter, the need for novel metallic alloys and alloy design are discussed. Definitions of HEA and CCA are detailed with the specific four core effects, high entropy, lattice distortions, sluggish diffusion, and cocktail effect. Then a quick overview is given on the mechanical and functional properties with a focus on hydrogen storage since CCA alloys for this application will be soon studied in parabolic flights. The chapter ends with the first thermophysical measurements carried out in parabolic flights on a FeCrNiCo-0.1Al alloy. The results of surface tension, viscosity, resistivity, sample radius, and mass density are reported showing in particular high viscosity. Along with interesting fundamental aspects, the perspectives of HEA and CCA for applications make essential to measure their thermophysical properties for the control of microstructure during the solidification stage of the processing. Acknowledgments The author thanks Dr. M. Mohr, Dr. R. K. Wunderlich, and Prof. H. J. Fecht from the Institute of Functional Nanosystems of Ulm University and members of the DLR ThermoLab and ESA ThermoProp core team for providing with thermophysical data on parabolic flights and fruitful discussions.

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26. C. Varvenne, A. Luque, W.A. Curtin, Theory of strengthening in fcc high entropy alloys. Acta Mater. 118, 164–176 (2016) 27. C. Varvenne, W.A. Curtin, Strengthening of high entropy alloys by dilute solute additions: CoCrFeNiAlx and CoCrFeNiMnAlx alloys. Scr. Mater. 138, 92–95 (2017) 28. E.P. George, W.A. Curtin, C.C. Tasan, High entropy alloys: A focused review of mechanical properties and deformation mechanisms. Acta Mater. 188, 435–474 (2020) 29. B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E.P. George, R.O. Ritchie, A fractureresistant high-entropy alloy for cryogenic applications. Science 345, 1153–1158 (2014) 30. X.H. Yan, Y. Zhang, Functional properties and promising applications of high entropy alloys. Scr. Mater. 187, 188–193 (2020) 31. L.J. Bannenberg, M. Heere, H. Benzidi, J. Montero, E.M. Dematteis, S. Suwarno, T. Jaron, M. Winny, P.A. Orlowski, W. Wegner, et al., Metal (boro-) hydrides for high energy density storage and relevant emerging technologies. Int. J. Hydrog. Energy 45, 33687–33730 (2020) 32. M. Hirscher, V.A. Yartys, M. Baricco, J.B. von Colbe, D. Blanchard, R.C. Bowman, D.P. Broom, C.E. Buckley, F. Chang, P. Chen, et al., Materials for hydrogen-based energy storage – Past, recent progress and future outlook. J. Alloys Compd. 827, 39 (2020) 33. K. Sakaki, H. Kim, K. Asano, Y. Nakamura, Hydrogen storage properties of Nb-based solid solution alloys with a BCC structure. J. Alloys Compd. 820, 6 (2020) 34. M.M. Nygard, G. Ek, D. Karlsson, M.H. Sorby, M. Sahlberg, B.C. Hauback, Counting electrons – A new approach to tailor the hydrogen sorption properties of high-entropy alloys. Acta Mater. 175, 121–129 (2019) 35. M.M. Nygard, G. Ek, D. Karlsson, M. Sahlberg, M.H. Sorby, B.C. Hauback, Hydrogen storage in high-entropy alloys with varying degree of local lattice strain. Int. J. Hydrog. Energy 44, 29140–29149 (2019) 36. I. Kunce, M. Polanski, J. Bystrzycki, Microstructure and hydrogen storage properties of a TiZrNbMoV high entropy alloy synthesized using Laser Engineered Net Shaping (LENS). Int. J. Hydrog. Energy 39, 9904–9910 (2014) 37. M. Sahlberg, D. Karlsson, C. Zlotea, U. Jansson, Superior hydrogen storage in high entropy alloys. Sci. Rep. 6, 6 (2016) 38. D. Karlsson, G. Ek, J. Cedervall, C. Zlotea, K.T. Moller, T.C. Hansen, J. Bednarcik, M. Paskevicius, M.H. Sorby, T.R. Jensen, et al., Structure and hydrogenation properties of a HfNbTiVZr high-entropy alloy. Inorg. Chem. 57, 2103–2110 (2018) 39. C. Zlotea, M.A. Sow, G. Ek, J.P. Couzinie, L. Perriere, I. Guillot, J. Bourgon, K.T. Moller, T.R. Jensen, E. Akiba, M. Sahlberg, Hydrogen sorption in TiZrNbHfTa high entropy alloy. J. Alloys Compd. 775, 667–674 (2019) 40. J. Montero, C. Zlotea, G. Ek, J.C. Crivello, L. Laversenne, M. Sahlberg, TiVZrNb multiprincipal-element alloy: Synthesis optimization, structural, and hydrogen sorption properties. Molecules 24, 14 (2019) 41. B. Hessel Silva, C. Zlotea, Y. Champion, W.J. Botta, G. Zepon, Design of TiVNb-(Cr, Ni or Co) multicomponent alloys with the same valence electron concentration for hydrogen storage. J. Alloys Compd. 865, 158767 (2021) 42. G. Zepon, D.R. Leiva, R.B. Strozi, A. Bedoch, S.J.A. Figueroa, T.T. Ishikawa, W.J. Botta, Hydrogen-induced phase transition of MgZrTiFe0.5Co0.5Ni0.5 high entropy alloy. Int. J. Hydrog. Energy 43, 1702–1708 (2018) 43. K.R. Cardoso, V. Roche, A.M. Jorge, F.J. Antiqueira, G. Zepon, Y. Champion, Hydrogen storage in MgAlTiFeNi high entropy alloy. J. Alloys Compd. 858, 9 (2021) 44. M. Mohr, H.J. Fecht, Investigating thermophysical properties under microgravity: A review. Adv. Eng. Mater. 23, 15 (2021) 45. V.V. V’Yukhin, O.A. Chikova, V.S. Tsepelev, Surface tension of liquid high-entropy equiatomic alloys of a Cu-Sn-Bi-In-Pb system. Russ. J. Phys. Chem. A 91, 613–616 (2017) 46. O.A. Chikova, K.Y. Shmakova, V.S. Tsepelev, Kinetic viscosity of molten high entropy alloys Cu–Sn–In–Bi–Pb. Izvestiya Vysshikh Uchebnykh Zavedenij Tsvetnaya Metallurgiya, 57–60 (2015)

Chapter 22

Laser-Assisted Additive Manufacturing of Ni-Based Superalloy Components Manoj Kumar, Jyotsna Dutta Majumdar, Hans-Jörg Fecht, and Indranil Manna

1 Engineering Materials The progress of human civilization is intimately related to discovering or developing new materials and exploiting the same for specific purpose. At the very beginning, this “new” possibly was as rudimentary as stone (harder, sharper, heavier, and more durable than clay or wood) in the Stone Age and as exotic as steel, glass, silicon, diamond, carbon nanotube, or cubic boron nitride in the modern times. No wonder why different era of early stages of human civilization is identified with specific solids (such as stone, copper, or iron) or the level of proficiency of utilization of them that made living better in some way or the other. History proves that a new material, compositionally or structurally, has often led to a crucial breakthrough in performance and functionality and enabled translation of innovative ideas and design into new components, devices, and machines and eventually opening up a new possibility (e.g., electrical machines and semiconductor devices) or greater level of efficiency (e.g., internal combustion and turbine engines). Materials science, engineering, and technology have been the key enabler in this journey as this

M. Kumar Metallurgical & Materials Engineering, IIT Kharagpur, Kharagpur, West Bengal, India CSIR-IMMT, Bhubaneswar, Odisha, India J. D. Majumdar Metallurgical & Materials Engineering, IIT Kharagpur, Kharagpur, West Bengal, India H.-J. Fecht Institute for FNS, University of Ulm, Ulm, Germany I. Manna (*) Metallurgical & Materials Engineering, IIT Kharagpur, Kharagpur, West Bengal, India BIT Mesra, Ranchi, India e-mail: [email protected] © The Minerals, Metals & Materials Society 2022 H.-J. Fecht, M. Mohr (eds.), Metallurgy in Space, The Minerals, Metals & Materials Series, https://doi.org/10.1007/978-3-030-89784-0_22

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seamless caucus has continuously endowed human society with improved standard of living and security against exigencies. Survival, growth, and prosperity of mankind, therefore, have hugely depended on the level of exploitation of engineering materials, be it for developing rudimentary tools or weapons in the pre-historic period to the most sophisticated components, devices, machines, and systems in the modern era. One must admit advancement in processing of materials plays a key role, too. All the solids materials may not be used for engineering application. The materials needed for any engineering purpose, naturally occurring or synthesized, and useful to fabricate an engineering component, device, appliance, structure, amenity, or a complete system can only qualify to be engineering materials. Imagination and innovation allow exploitation of engineering materials to design and develop a new and novel solution that either nature does not offer as a ready solution or does not exist as yet. Water that ensures existence of life on earth is not an engineering material, but steam from water needed to generate electricity or locomotion certainly qualifies as engineering material. Frankly, almost all matter in any physical state can offer some utility and hence qualify as engineering materials. Undoubtedly, metals should head that list not just for historical legacy but purely because of the sheer volume of application and utility. From the oldest metal artifact, a copper awl [1] (a drill or a conical tool) unearthed in Tel Tsaf village of Israel dating more than 7000 years, all the way to the most wonderful and useful metal of modern era, the superalloys (material for aeroengine turbine blades) that has survived even after sustained use in the most challenging conditions and has still been evolving since its earliest formulation nearly a century ago, the history of metals or more precisely metallic alloys is replete with an amazing number and fascinating documentary of development over centuries. Let us take superalloy as an example.

2 History of Superalloy All crystalline solids, pure element or multicomponent alloys, soften as they approach and eventually melt or liquefy above the respective melting, fusion, or liquidus temperatures. Retaining strength at elevated temperatures, not necessarily close to fusion but even above half of its melting temperature, is an ambition that alloy designers have always keenly pursued. This challenge arises because strengthening mechanism of crystalline solids undergoes a major change from boundary dominated to grain body-controlled regime at such high temperatures. The principal motivation for developing alloys with high-temperature strength comes from the applications that demand operating at high temperature for the sake of enhancing efficiency, conducting specific jobs or functions, and exploring operations not feasible at ambient conditions. In case of steel, the most widely exploited workhorse metallic solid used for structural applications has undergone a series of experimentation over the years with the primary objective of retaining strength at elevated temperature. The outcome was evolution of a series of structural materials like alloy

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steels, tools steels, stainless steels, high-speed steels, maraging steels, and superalloys, though the last two being technically more appropriate to be called nonferrous alloys than steels going by composition. Superalloys are based on a multicomponent recipe with nickel, nickel-iron, or cobalt as the main or matrix element, offering excellent combination of mechanical strength, creep, and fatigue properties and resistance to oxidation and corrosion at elevated temperature. These heat-resisting alloys are practically the ultimate choice for aircraft jet engines, gas turbines, chemical process industries, heat exchanger tubes, nuclear reactors, coal conversion and gasification plants, and all other applications operating at temperatures well above half of their melting/fusion temperatures [2]. Superalloys are classified into three groups based on the major or matrix element present, namely, (a) Fe-based superalloys, (b) Ni-based superalloys, and (c) Co-based superalloys. The usual alloying elements include Cr, Al, Ti, Nb, V, Mo, etc. Fe- or Fe+Ni-based superalloys with up to 25 wt.% Ni are essentially austenitic or face-centered cubic (FCC) matrix alloys strengthened both by solid solution and precipitation hardening that offer good mechanical strength at room temperature as well as at elevated temperature [2]. These alloys are also characterized by resistance to oxidation, wear, creep, and hot corrosion. Fe-based superalloys are relatively cheaper compared to Ni- or Co-based superalloys. On the other hand, Ni-based superalloys are the most widely used in the hottest parts of turbine and similar applications. More than 50% by weight of the aircraft engine components are made from Ni-based superalloys due to greater phase stability of Ni-rich FCC matrix and associated ordered precipitates that arise following precipitation hardening [2]. Presence of alloying elements like Cr and Al readily enhances the corrosion and oxidation resistance. Ni-based superalloys usually retain strength up to about 1100
C. In comparison, Co-based superalloys can retain their strength at even higher temperature, though offering strength slightly inferior to that of Ni-based superalloys. Presence of refractory metal carbides (e.g., WC, Mo2C) along grain boundaries is the main reason for retaining strength at higher, nearly close to melting temperatures in these alloys. Higher level of Cr makes these alloys more corrosion and oxidation resistant (through formation of Cr2O3 film on the surface). Co-based superalloys are also fairly thermal shock resistant. Inconel series of alloys, say Inconel 718 (55Ni-21Cr-5Nb-3Mo), belong to Ni-Crbased superalloys and are available in a wide composition range (containing varying amounts of Fe, Al, Nb, Ti, and Mo). These superalloys offer very attractive combination of mechanical properties mainly due to solid solution and precipitation hardening. The key allotriomorphic or idiomorphic precipitates in the Ni-rich γ matrix (FCC) are γ00 (Ni3Nb) and γ0 (Ni3TiAl), along with some amounts of interstitial carbide compounds of Ti or Nb, despite having very low C content. Mo enhances solid solution hardening. Inconel 718 (IN718) is very effective in registering steady diffusion-controlled parabolic-rate oxidation resistance at atmospheric pressure in air over a large temperature range (950–1300
C) [3]. At still higher temperature, strength deteriorates rapidly and oxidation is almost catastrophic

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[4]. Inconel alloys are suitable for several applications like nuclear reactors, aero/gas turbines, and rocket motors, as already stated, because of significant resistance to creep, thermal fatigue, oxidation, hot corrosion, and erosion. Further extension of life and reliability is often ensured at elevated temperatures by providing a protective layer at the surface, called thermal barrier coating [5].

3 Laser Additive Manufacturing (LAM) Additive manufacturing is an innovative approach based on sequential integration of microscale melting and solidification events in one-, two-, and three-dimensional space following a definite design and geometry. Laser is useful for a wide variety of materials processing like cutting, joining, fabricating, surface engineering, and repairing of components [5]. As a logical extension, laser-assisted additive manufacturing (LAM) has now emerged as the most versatile, flexible, and effective method of additive manufacturing widely applied for direct manufacturing of finished net shape or near-net shape metallic components with the aid of computeraided design tools. LAM adopts two main approaches: laser-based powder bed fusion (LPBF) and laser-assisted direct energy deposition (LDED) techniques [6]. LPBF process may be conducted by selective laser melting (SLM), selective laser sintering (SLS), or laser metal fusion (LMF) methods, while LDED may involve laser-engineered net shaping (LENS), direct metal deposition (DMD), or laser metal deposition (LMD) methods, respectively [7]. Historically, the concept of additive manufacturing including LAM can be considered as an extension of rapid prototyping of polymeric materials and rapid tooling technology of metallic components [8]. The process sequence in additive manufacturing involves computer-aided design (CAD) of the component ! creation of three-dimensional (3D) view with two-dimensional (2D) slices in standard template library of digital files ! transfer of design to LAM machine console ! development and adoption of manufacturing routine ! cleaning and withdrawal from the powder bed in the machine ! post processing operations, if any. In general, additive manufacturing process may be classified into the following categories: vat photo-polymerization, binder jetting, materials jetting, sheet lamination, materials extrusion, powder bed fusing, and direct energy deposition [9]. As narrated above, the final product, miniature or large, is never made all in one single operation like metal casting or ceramic powder sintering but in multiple identical steps with provisions for changing the composition, process parameters, and accompanying thermal history. In simple words, like in calculus, integration is a mathematical process of joining multiple small differentials, and in engineering, additive manufacturing is a similar physical process of integration of infinitesimally small steps that eventually delivers a finished product. The overall scope and novelty of LAM lies not just in producing a finished product of intricate and novel geometry and functionality but also in developing a product with microstructure, phase aggregate, and properties quite different from what is expected from an engineering

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solid fabricated by conventional processing techniques of melting, solidification, sintering, or deposition. Imagine a machine tool, a bone or dental implant, a metallic valve or nozzle, or an expensive die that needs a change in design for customized fabrication or repair after severe wear and tear. It will not be an easy proposition by conventional route even if it appears simple. This is because conventional manufacturing offers limited freedom to fabricate components of complex shapes, change in dimensions or geometry, vary raw material and composition, and repair a worn or broken component with a fracture surface of irregular or random contour. Even if such improvisation is possible, it will not be economical or convenient, particularly when the number or volume of product or cost per component is small. However, change in shape, dimension, contour, and even composition, partially or wholly, may be necessary, that too in small volumes or quantity say, for a human patient with cancer-afflicted bone or a brake pad or bearing of an expensive racing car. In this respect, additive manufacturing, popularly called 3D printing, is a very attractive proposition for direct fabrication of a wide range of solid structures with variable size, shape, geometry, and complexity from the corresponding 3D CAD model in terms of flexibility, precision, cost, space, time, and novelty. As already explained, the process of LAM will simply involve integration of droplets and successive deposition or printing of successive layers as per the predetermined design. The pioneering effort of developing such 3D products of polymers by Charles Hull in 1986 using stereo-lithography (SLA) of polymeric precursor was the earliest precursor of additive manufacturing [10]. Though the concept of additive manufacturing using polymers may be fairly old, the exponential growth in the interest and application of this processing route, not just for prototyping but for direct manufacturing of engineering components and products using metallic alloys, is barely a decade old. Once proven feasible, use of metallic powders to develop metallic components was vigorously pursued by powder bed fusion, fused deposition modelling, inkjet printing, and contour crafting approaches. Obviously, application of laser has made the process much more versatile and precise. As a result, LAM is now adopted in almost all sectors of engineering like metal manufacturing, aerospace, automobile, defense, electronics, civil construction, biomedical prosthesis, and many more. Some of the potential benefits and novelty of LAM products can be (a) direct translation of design to finished product with minimum lead or development time; (b) direct manufacturing of components to final or near-net dimension and shape with minimal or no additional processing; (c) scope of greater customization with no additional tooling or manufacturing cost; (d) introduction of design novelty of retaining hollow volume, complex contour, controlled porosity, and special internal features; (e) approach to a practically zero-waste manufacturing practice; (f) improvisation on demand with no or minimal cost or downtime; and (g) linear scalability. However, it is important to note that LAM products do suffer from inherent flaws, deficiencies, and defects ranging from porosity, cracks, shrinkage, inclusion, contamination, segregation, surface roughness, distortion, and poor mechanical properties as compared to those achievable by conventional practices.

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Established components developed and utilized in large scale by additive manufacturing of metals are mostly based on various grades of steel and aluminum alloys for limited structural (load-bearing) and functional (non-load-bearing) applications in ambient condition. Since most components made by additive manufacturing possess metastable microstructure which may transform toward equilibrium state when exposed to high temperature for extended period continuously or in cycles, application of additively manufacture components at elevated temperature is rare and sparsely explored. However, superalloys are meant for prolong use at elevated temperature under constant or cyclic loading condition. Presence of aggressive environment like air, dust, and combustion products demands even more rugged resistance against environmental and mechanical degradation. Thus, studies on development and evaluation of performance and reliability of additively manufactured superalloy components are warranted before fresh attempts are made to employ superalloy-based components in hitherto unexplored critical applications and even substitute the conventional products.

4 Additive Manufacturing of Ni-Based Superalloys As a group of Ni-/Co-/Fe-based multicomponent alloys with a unique ability to retain mechanical strength at temperatures close to melting or fusion temperature, superalloys find wide and ever-increasing application for key structural components in aviation, aerospace, and power generation industry. Melting to final fabrication of superalloy component requires elaborate, multistage, expensive, and timeconsuming processes that is neither common nor easy to practice. Single crystal or even directionally solidified Ni-based superalloy blades for aero-turbine engine are possibly one of most sophisticated and challenging material processing exercises that can easily be cited as a triumph of material technology of recent times. For the advantage of shortening the production time, minimizing the material waste and manufacturing cost, additive manufacturing is gradually becoming the most sought tool for development and repairing of components in various sectors like aerospace, automotive, and medical sectors. In particular, LAM provides the flexibility to fabricate components with controlled microstructure, composition, and texture with minimum defects by proper selection of laser parameters. The extremely rapid cooling rate associated with LAM can create novel and finer microstructure with minimal segregation that can compete with superalloy components developed by conventional methodology of melting and casting. Despite significant success in recent times, the technology concerning LAM is far from being matured enough to foresee the possibility of LAM altogether replacing conventional processing route to develop the entire gamut of all superalloy components currently in use in various industry sectors. In order to understand the current status of developing and exploiting superalloy-based engineering components by LAM and hurdles faced or anticipated by the user industry, an attempt will now be made to review the literature and examine the key advancements and challenges with regard to

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microstructural evolution, mechanical properties, dimensional compliance, defects, processing difficulties, reliability, and life assessment.

4.1

Powder Bed Processing by LAM

Broadly, LAM can be conducted either by fixed powder bed or loose powder fed systems where the laser-matter interaction in micro-millisecond period allows melting and solidification in a small volume. As Fig. 22.1 shows, this process in a typical powder bed system creates a solid layer of limited depth and continues in directions guided by the computer-aided design that drives the laser beam [11]. Each new layer on solidification welds with the preceding one laterally and vertically. The phase aggregate of this layer will depend on the laser (wavelength, beam profile, and intensity), process parameters (power density, interaction time/speed, focus position, powder composition, size, shape, and packing density), and material properties (melting/fusion point, specific heat, density, surface energy, thermal conductivity, reflectivity, absorptivity). The resultant microstructure for a given powder bed composition is a complex function of the thermophysical parameters like heating/ cooling rate, thermal gradient, melt pool geometry and convection, etc. The main handle to control the microstructure and properties of the product developed by LAM are the two independent process parameters, namely, laser power density and interaction time. 1: Convection liquid/gas 2: Liquid pool radiation 3: Solidified part radiation 4: Convection solid/gas 5: Conduction liquid/solid 6: Conduction liquid/powder 7: Evaporation 8: Convection 9: Surface tension g: gravity Gas

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Fig. 22.1 Schematic of various physical phenomenon taking place in SLM process during laser interaction with pre-sprayed powder bed [11]

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Effect of Process Parameters

Selective laser melting (SLM) and direct energy deposition (DED) are the two major techniques for additive manufacturing of metals and alloys. Jia and Gu [4] have shown that inadequate power density tends to spheroidize or ball up powder particles and reduces relative density of the solid. A reasonable power density is needed for near-full densification. The typical microstructures of SLM-processed IN718 parts may manifest morphological changes from coarsened columnar dendrites to clustered dendrites or even thin and uniformly distributed columnar dendrites. This resulted in uniform microhardness distribution, reduced wear rate, and low friction coefficient [12]. Xia et al. [13] have reported that scanning speed or interaction time largely determines the size, shape, and distribution of porosities in the SLM-processed IN718 components. It appears that open porosities form on the surface and also between the layers when laser is processed with a high scanning speed of 500 mm/s due to limited or incomplete melting and wetting of powder. In contrast, laser processing with relatively lower scanning speed of 200 mm/s results in formation of smooth surface with lower porosity on the top surface and cross-sectional plane indicating good metallurgical bonding between successive layers in the growth direction. Figure 22.2 shows a process or weldability map for IN718 as a function of the independent process parameters of effective laser power and beam velocity [14]. It is apparent that an appropriate choice of laser power and scan speed is essential for

Effective Powder / W

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